It depends on the store and parts availability. Independent Apple Authorized Service Providers tend to do the top case replacement because they apparently do not have the unsticking equipment that Apple has deployed in stores..
Then there is price. Apple publishes a fixed price fo US$199 for replace the battery and will honor that even if they end up replacing the entire upper case for their convenience. An independent who does the upper case change will probably charge US$500-600 based on too many reports here.
Also your concern only applies to Retina display models. The non-Retina 2012s have the battery held in with screws behind an easily removable bottom cover.
However, the big issue is "2012." Unless your MBP is the 13-inch non-Retina model, all other 2102 MBPs are considered vintage and generally Apple Store will not do the work and you will be seeking other options.
I hope you have a non-Retina model.