In reply to AndyMusgrove:
Hi Andy, I would be surprised if you need a 34 on the back - my gravel bike has a 34/34 lowest gear and I only use that on insane steep off-road sections.
Generally for equipment from 2015 - I would assume if you have a medium rear cage that would accommodate a 32 max (or if a short cage 28). Newer rear derailleurs (R7000 etc onwards) can have a max 34 on the back. 34/32 should still be fine for any road.
With regards to changing cassette, as someone has said would be good idea to change the chain as well. Good test is try to pinch the chain at the front of the chainring and pull it forwards, if it feels taught its ok but if its relatively easy to pull away a small gap (a few mm) then the chain will be stretched. A stretched chain will make mincemeat of a new cassette and will not run well (as its likely your existing cassette will have had its teeth "modified" by the stretched chain). Generally I go through ~3 chains before cassette replacement, front chainrings should last a lot longer.