In my personal case I went to tribunal because they lowered my higher rate mobility by two points so did not qualify however at the time of the DLA to PIP review they increased my middle rate care to higher rate care.
When I went down the tribunal route I was warned they could revoke the higher rate care while still not qualifying for higher rate mobility but everyone I spoke to who were in the know as it were said there was no way I’d lose the higher rate care and 99% chance I’d win my higher rate mobility award.
It’s a risk for sure and something you need to think about but like MFillingham and others have said I think there less likely to change the part your not challenging and at my tribunal there was no mention of my care award and they were only concerned with the facts and why I think the DWP got it wrong with the mobility part and why I think I’m right.
At the tribunal I won the higher rate mobility and kept the higher rate care along with a 10 year award as the judge said that I should be awarded for an “on going period” as I’m unlikely to get any better after he asked how long had I been in my current position and I told him 10 years as it was at that time.