Getting from UC back to IS

I know this has been covered before (almost certainly more than once) but I’d be grateful if someone could steer me (possibly just by directing me to discussion where this has been covered before).

Couple with
Partner 1 receives PIP and is HB claimant
Partner 2 (who is younger)  receives IS and CA
They also get CTR.

When Partner 1 reaches pension age and starts to get SP.
Partner 2 then gets IS revised to include pensioner premium with deduction for the SP.
Because younger partner still receives IS the couple are entitled to continue to receive HB (under the fiction that partner 1 is treated as still working age even though they are not). I’m fairly confident that is the case although partner 1 had letter from council saying HB will end when Partner 1 reaches pension age.

I am more concerned about the following.
Partner 1 is due PIP review. If PIP is lost Partner 2 will lose CA and IS. If this happens after Partner 1 is pension age HB will also end. Faced with no income or just Partner 1’s SP they will then have to claim UC in order to survive.

My question is, if PIP is subsequently reinstated is there any way back onto IS?

Have just realised that my question is probably covered in this thread https://www.rightsnet.org.uk/forums/viewthread/15312/
My conclusion is that it should be possible to get IS back in place from the date it ended but probably only up to the date UC was claimed and to actually reverse out of UC (but I think I may need to read the thread at the start of the day rather than the end!).