features, bug fixes, compatibilities, functionality...I'm not going to detail everything that is in firmware release notes from last 3 years.
Upgrades need thinking about before jumping because they can have unintended consequences and that's your responsibility to plan ahead and avoid. Might take a few hours but look at release notes for applicable firmwares and decide if all those things are relevant, useful or desirable then decide.
Possibilities;
1. Stick with present ZD11/7363 and move to highest firmware (after careful consideration) that both support - 9.10.2.0 You will get some fixes, features, enhancements but not latest and greatest.
2. Buy ZD12 for better control and faster reporting but accept limitations imposed by EOF 7363. Upgrade to firmware as in 1 will give benefits.
3. Wait until you can afford to replace both ZD and APs. Save your money for now and plan sensibly for future upgrade.
Any solution that purchases a ZD12 but trys to include the old ZD11 in the mix is over complicating your upgrade. You are already limited by your older EOF 7363APs as to firmware you can upgrade to anyway.
A ZD1200 will handle your present load much better than ZD11. It is a far more capable device than the ZD11. If your present client load/guest generation is approaching limits on ZD11 then the ZD12 will make everything much smoother.