The Wap Ac are MIPSBE, I believe:
So you will need to run the two CAPSMANs (old and new), which is possible but obviously a bit more complex.
I don’t know, but if your current hex is working, you could, instead of replacing it, keep it besides the new hex s, it depends on how your network layout is, it could be easier to maintain.
The CRS-354’s are running RouterOS, right?
So yes, you can update them, to have everything on a same release.
A good question is which release to update to.
From various reports on the Forum, I believe that the current 7.20 is still a bit rough around the edges, so right now I would go for 7.19.4 or 7.19.6 or - if you are more conservative, 7.15.4.