I don't see a compatible firmware that supports both 7363s and r510s. The only way I can think of to avoid downtime is to sever the connection from 7363 radios and the 1200 on firmware 9.12 to force them to operate autonomously until I connect a new AP and repair the connection, with newer firmware on the ZD. I'll probably do it with VLANs. Would this work, and if so, for how long will the 7363s continue to operate?
not sure what features you're using on the 7363 (like radius / 802.1x auth, dpsk, etc). Very few features allow the AP to work in "survivability" mode. You might have better luck disconnecting those APs and loading the stand-alone firmware on them and running them stand-alone (if the features needed are available on stand-alone). Then upgrade the system to allow the R510s to join and swap the APs one by one?
The 7363 is supported thru 9.12.x releases, but R510 begins support on 9.13 GA. You can upgrade your ZD1200 for the R510s, but might need to run Solo AP firmware on the 7363s. Best if not next to your ZD controlled APs, as re-authentication is required when clients roam.
Probably not long. The timer is set in your ZD. AP will reboot if it cannot contact the ZD, by default in 30 min. You may extend that time, but in most cases, if there is authentication on your WLANs, the wifi will fail almost immediately, since authentication will fail. I would hang the new APs. They will not be able to connect to the ZD, because they will not be able to downgrade to the ZD version. Old APs would continue to work. Upgrade the ZD. Now the new APs will start connecting and taking config, while the old ones will fail. That's all depends if the license on your ZD supports the two types of APs and their numbers 😞
I'm not sure how many APs you have, but you could keep the ZD on 9.12 and convert the R510 APs to Unleashed. You can maintain current functionality on the 7363 and have close to full ZD functionality on the R510s (25 APs max, 512 users max, no DPSK).
Another possibility is to search ebay for a cheap ZD1100 (preferred) or even a ZD1000 with enough licenses and use that for the 7363 APs. Downgrade the code to 9.7.2 if ZD1100 or 9.3.4 if ZD1000. Then you can upgrade the 1200 9.13 or higher for the R510s. The right long term answer is to replace the 7363 APs with a newer model and put everything on the ZD1200.