Change of circumstances while waiting for a PIP decision?

I submitted my PIP renewal back in March and am still waiting for a decision and it's now December! I get a text every two months telling me it's still being processed and that they could still call me for an assessment so at least I know I'm still in the queue!
My question is, should I notify them of an improvement in my condition now or wait until they've finally made a decision?
I have Relapsing Remitting MS and for the last few months the I've been able to walk a bit further than I could back in March which would put me in a different category for mobility.
Any guidance much appreciated!

Thanks for your reply. I had intended to wait until they'd made a decision but it's now been 9 months since I sent my renewal in and I'm still waiting!
Are these kinds of delays common at the moment?
If I inform them of a change in circumstances now will they want me to start the PIP renewal all over again and will they continue paying me at the current rate while they process the change?
I'm also worried that if I don't tell them about a change they might claim some of my PIP payments back.
Basically I don't understand the process so any clarity would be much appreciated!