UA does not do automatic free upgrades for anyone, not even their own elite members.

When you check in you may be offered the opportunity to "buy up" to first class. Make sure you ask when you check if in you're interested.

*A Silver and *A Gold members are no longer eligible to sit in economy plus, so you'll be sitting in regular coach. There is anecdotal evidence that many gate agents will upgrade *A elite member to E+ for free, but this is the exception and not the rule.

You'll still get priority boarding and priority check in (depending on the airport - sometimes UA limits priority check in only to *A Gold members).

Also, if you purchased your UA ticket prior to hitting Silver in DM, call UA and have them delete your DM number and add it back in. Otherwise there's a good chance your PNR will not reflect that you are *A silver and you'll lose the paltry benefits that you would have gotten.