I'm tempted to do this, I'd be interested to hear how you get on, and if your able to get OTA sense updates after changing back but keeping root and S-off.
OK, well last night I gave it a go and am pleased to report that it seems to have worked. Although I don't know whether it'll accept OTA updates, I am running Sense 6.0 with Lollipop.
I followed the instructions here and flashed the Sense firmware package: http://forum.xda-developers.com/showthread.php?t=2733523
I then tried to reboot, as per instructions, but the phone would not boot. It just sat there on the HTC screen and then rebooted. Had to hold power + volume up to hard reboot, and then volume up to enter HBOOT. Couldn't work out what I was supposed to do next as the instructions were not clear. I eventually realised that switching from HBOOT to FASTBOOT whilst USB was connected would reboot the phone, but if I removed the cable and switched to FASTBOOT, then put the cable back the PC would recognise the phone. That meant I could run FASTBOOT commands. I tried flashing TWRP as per the thread but the phone wouldn't load it. I thought this might be because my firmware was too old, so flashed an earlier version of TWRP but no dice.
Then I thought - why don't I just load RUU mode and use the Lollipop RUU from here? http://forum.xda-developers.com/htc-one-m8/general/guide-update-to-official-developer-t3002003 I was rather confused and didn't know whether I could or not, but it worked perfectly and now I am up and running with Lollipop.
So there we go, not 100% sure what I did was correct but it does seem to work. Will report back with any findings. As for Lollipop/Sense 6.0 it really is more like they ported Lollipop to Sense, rather than the other way round. It runs well but the visual changes are few so don't do it expecting lots of big changes. Still, I have Lollipop Sense and that is good I expect Sense 7.0 will be out in the next couple of months.