You can go from v1.10.
The one issue is that with v1.20, it encrypts the FW on the router. If WRT is applied with v1.20, WRT won't load correctly. So I found that going back to v1.1x or prior, WRT will load.
I have not tried recent versions of WRT lately so they may have fixed this load issue going from v1.20 to WRT. Maybe you can ask someone there and see. Loading WRT on v1.10 will work fine.