I swapped the battery a year or two ago after the original one wasn't holding a charge. Then the replacement started bulging after I left it in the sun and connected to the charger while on a road trip (whoops!) Bulging battery is a fire hazard, and the price was right vs the price of a new flagship phone, so repair was a no-brainer.
Super easy, given that this wasn't my first battery swap in this phone. I didn't bother putting in the new adhesive border the last time I did the battery swap, since my case held the screen on relatively securely along with the remaining adhesive that was still good. I removed the screen with my thumbnail.
If you don't care about water tightness and your case holds the screen on anyway, don't bother with the adhesive. If the phone is old enough, it's not worth the hassle.