Looking at random dates for BOS-JFK. For 1 passenger, I see BE. However when I select 2 passengers, BE is no longer an option.

In theory, no, no changes. Would kind of defeat the purpose for AA if you could just pay the fare difference and then have a changeable fare. I think if you call to upfare within 24 hours you'll have better luck, but generally I'd guess that YMMV.
That said, most employers, particularly those using corporate travel agents/platforms, have blocked BE fares from even showing, because of the non-changeability.

AA’s transatlantic Joint Business Venture Partners AY, BA, IB are implementing AA Basic Economy restrictions on their airlines in conjunction with AA, at least in large part, in April 2018. This will prevent AA flyers from taking up the cheapest fares on Y, BA and IB to avoid some of the AA Basic Economy restrictions on baggage, changes, miles earning, etc. The AA information and links have been added to the Wikipost at the top of the page.

American and Partners Imposing Basic Economy Restrictions on Cheapest Transatlantic Fares Next Month by Gary Leff on March 1, 2018 <link>

In part:

But, #2 has nothing to do with Basic Economy
AA elites wouldn't get lounge access for this route even on a full F ticket

Starting in April, we will introduce Basic Economy on some trans-Atlantic flights, giving customers a new option for our lowest fares.

We're making this addition together with our Atlantic Joint Business partners British Airways, Iberia and Finnair.

We're following the same playbook in this latest expansion as we did for the initial rollout of Basic Economy. That includes training that began this week for Airports and Reservations team members and making it available in a limited number of markets first to make sure we get it right.

In general, most options and restrictions for trans-Atlantic Basic Economy are similar to those for domestic Basic Economy or for short-haul international Basic Economy such as flights to Canada, although there are a few differences.

A closer look at domestic and trans-Atlantic Basic Economy

https://newjetnet.aa.com/servlet/Jiv...Info-Sheet.png
Click to enlarge imageWe'll have more to share about trans-Atlantic Basic Economy when it launches in April. For now, here's a preview of how it will work:

Boarding - Trans-Atlantic Basic Economy customers, including those originating with a domestic leg, will board in Group 8. Elite customers and eligible AAdvantage® credit card members will continue to receive Priority or preferred boarding even when purchasing this fare.

Carry-on Bags - The carry-on bag allowance for all international Main Cabin fares, including trans-Atlantic Basic Economy, is one personal item and one larger carry-on. This is unchanged from international Main Cabin fares today.

Checked Bags - Regular Main Cabin fares will continue to include one checked bag for free. A new fee will apply for the first checked bag on trans-Atlantic Basic Economy.

Connections - Customers flying on a domestic Basic Economy leg connecting to a trans-Atlantic Basic Economy ticket will travel under the rules of the international ticket, including the carry-on bag allowance.

Inflight experience - Regardless of whether they are traveling on a Basic Economy fare or a regular Main Cabin fare, all Main Cabin customers will have the same experience, including the same free entertainment, soft drinks, snacks and meals offered today.

Seat assignments - Free seat assignments are made automatically when customers check in. Customers flying trans-Atlantic Basic Economy can purchase a seat assignment at any time.

Tickets - Non-refundable. No same-day flight change or same-day standby. However, the tickets are changeable (for a fee), which is different from domestic Basic Economy where changes are not allowed at all.

Upgrades - Not permitted, regardless of elite status level.

I’m flying on my first Basic Economy ticket right now from DFW-BWI. $87 for a last minute one way vs. $332 for Main Cabin. With AA Gold status, it hasn’t felt that different than a regular coach fare, aside from the inability to upgrade (not that I would have cleared the upgrade list anyway with my status!)

I declined to pay for an advance seat assignment and was given 25E when I checked in on the AA app last night. However, an Admirals Club agent changed me to an exit row aisle at D-45 when I asked, no charge. I imagine the seat policy is a YMMV issue, but I was pleased. Exit row would have cost me $50 on this fare if I had picked it at check in.

Yeah, this just narrowed one of the biggest gaps between AA and UA's 100K benefits. I had always disliked the idea of having to buy higher economy fares to use a GPU, and that was one of the biggest factors that kept me with AA when all the cuts were made a couple years ago.

Not that I will jump ship now, but it certainly levels the SWU-usage benefit we've had with AA over UA all along.

I'm flying AA in B class for the first time. I'm AAdvantage Platinum and flying on Tuesday from EWR to MIA. I see that it's going to snow in NYC on Tuesday. What's the best strategy to get a shot at being rebooked out of B class? Is it best to call? See people at the Admirals Club (I have the Citi Executive Card), or gate/ticking area? I know that they're probably supposed to oversell B on whatever flight I end up on, but I assume someone might just pick whatever fare class is available (ideally Y, so I get 1.5 EQM, every little bit of ~500 EQM counts).

I am having a hard time locating here if AA platinums can do a free same day flight change on a basic economy fare. Can anyone assist?

The answer is no:

http://s21.q4cdn.com/616071541/files...info-sheet.png

Newsroom - Coming Soon: Basic Economy on Select Flights to Europe - American Airlines Group, Inc.

Ok, I saw that. I just thought there may have been an exemption for elites.

The exception for elites is marked with a an asterisk against carry-on bags. There is none so marked in relation to same day flight changes or standby.

I am not so sure the price difference will be huge. Even now, in most instances (there are certainly exceptions), the price of basic economy isn't all that much lower than standard fares.

And from my understanding, the airlines don't want you to buy basic economy fares unless you are very price conscious. They have to price standard fares at a premium that is only commensurate with the benefits, if they want people to buy it.