R81.20 does udev rules just like all R80.10+ versions.
[Expert@vsxMember1:0 ACTIVE]# fw ver
This is Check Point's software version R81.20 - Build 012
[Expert@vsxMember1:0 ACTIVE]# cat /etc/udev/rules.d/00-OS-XX.rules
ID=="0000:04:00.0", NAME="eth0"
ID=="0000:04:00.1", NAME="eth1"
ID=="0000:04:00.2", NAME="eth2"
ID=="0000:04:00.3", NAME="eth3"
ID=="0000:02:00.0", NAME="eth4"
ID=="0000:02:00.1", NAME="eth5"
ID=="0000:08:00.0", NAME="eth6"
ID=="0000:08:00.1", NAME="eth7"
ID=="0000:05:00.0", NAME="eth8"
ID=="0000:05:00.1", NAME="eth9"
ID=="0000:02:00.2", NAME="eth10"
ID=="0000:02:00.3", NAME="eth11"
I actually took this exact cluster from R80.40 directly to R81.20. It worked fine.
I don't remember when we set up VSLS, but that's pretty easy to do. May as well enable it on R80.40 before the upgrade.