Also curious about this. Please report back!

with the caveat that I have not done this personally, I was told by multiple reps that it is possible to cancel an existing booking once you have a lifemiles+ subscription. I was also told that it takes 30 days from the time you subscribe for the subscription to take affect

The subscription I think takes effect instantly?

No. They see your LM+ join date is after your ticket issued date.

You're better off waiting for a possible schedule change to cancel for free.

After you make payment you'll get a webpage that says thank you for payment and you enjoy the benefits of the subscription from payment date.

As an aside, I have LM+ Lite from the AMEX Cardless Elite card. I paid on Nov 28 for LM+ Basic because I wanted to book a few flights with the ability to cancel for free (free cancellation of LifeMiles redemeptions is only possible with the Basic plan or higher). I got charged $30 on my AMEX Avianca card and got the website that I noted above - so I felt comfortable buying the ticket shortly after subscribing. I waited for many days but my LM+ plan page never changed to Basic, it stayed on "Lite" and I received 500 miles on Dec 4, which the day I receive my monthly LM+ miles benefit. I did not receive 2000 miles as is promised with the Basic plan. I called LifeMiles and they said to wait 30 days for it to be reflected on my account. I waited until Dec 28 - nothing changed. Suffice it to say I am still waiting to this day for resolution - they have a request in the system that they have not done anything with. At this point I don't care if it is a refund of my $30 or a retroactive application of the status. I just want resolution! I keep calling to get it bumped up in priority.

I wouldn't know. I paid $30 to upgrade (on the Avianca Cardless card, mind you) to the Basic plan on November 28 and to this day my plan never changed from Lite. I have an open request with LifeMiles to get this fixed (the request has been in the system for a little over a month now), be it a refund or a retroactive application of the status I don't care at this point.

Do they refund the $25 partner booking fee also if you have LM+?

It's non-refundable unless invol sched chg.

When booking two awards from MEX to FRA I am offered a 10% discount (72k instead of 80k miles) if I join Lifemiles+. I am wondering if this discount is only for my ticket or also for other pax on the same reservation? And does anybody know if the discount is available immediately after signing up or does it take some time? Thanks

Discount applies on all passengers on the reservation. You get instant access to the discount upon payment. Keep in mind lifemiles+ has a min. 6 months subscription although there are some reports members got away paying it and didn't get their accounts suspended (yet).

Does it take some time to process / reflect that you have subscribed to LM +

I think I may have subscribed twice ( did the basic , 2x $50 charges on my card)

Still does not show any discount when I try to purchase tickets.

(so I guess contrary to what others have said, my subscription is not showing up instantly)

All discounts associated with LM+ are baked into the mileage you see. If you want to see what kind of discount you are getting, you need to create an account without LM+ to see the non-discounted mileage. I use my daughter's account to see the non-discounted price and use that information to get an idea of how much money I'm saving with LM+.

Generally, for most flights it is 10% on the dot. Recently, Avianca jacked up all the mileage rates for no reason to ridiculous mileage rates - what was 9k to get to Bogota jumped to 17k. I knew it was temporary as it always is, and sure enough the mileage came back down to 9k, but in that time it was 17k for the route I was interested in, the discount was 25% (LM+ bills itself as offering 10-25% discount). Well 25% off 17k is only 14k. I'd rather get 10% off 9k and pay less mileage than get 25% 17k.

No partner booking fee refunds
 

Beware as they don’t refund the $25 partner booking fee and this can substantially add up.

I got a targeted promo in my email for a 1000 mile bonus when joining the lowest tier of LM+. The terms give a period between May 22 and June 11.

That's not bad. Just remember that you are required to pay for 6 months. I would only suggest you do it if you have flights to redeem coming up so you can maximize that sweet 10% savings.. That's what I did with Club LifeMiles (the predecessor to lm+) last year. I had a flight in July and my annual Christmas flights that I wanted to get. I saved 6300 miles for like $430 of flight value.

Also, if you do it, make sure to take a screenshot of the confirmation page after paying.