CFW 4.21 -> CFW higher/lower firmware --- Possible?

All this talk about the PUP only being signed to be installed om 3.55 firmwares and that it's impossible to upgrade/downgrade higher firmwares got me thinking.
If I upgrade to CFW 4.21, will I even be able to install any other CFWs from it, or will I be stuck until they release the keys for 4.21?

I'm not fully informed about this mess with keys and stuff, so I thought it's safer to ask before I do something drastic.
Are there any cons with upgrading to the new CFW, except for that 3.55 homebrew/psn games wont work and the slight compatibility issues (probably gone with v1.09?)