No set rule, so long as the company know of any change you're covered, unless of course when telling them they said no we won't cover you.
Why not try Ultraseal, had it in my cossie and gf's astra best thing I did, tyre life is around 25% better most likely due to them running cooler, if you notice a nail just wait until you are going out in the car pull the nail out and drive off, job done.
I got a 8mm bolt stuck in a tyre, looked like a ford gearbox bolt, I was astounded it sealed this, despite being advertised up to 5mm seal, this is a permanent fix no taking tyre off etc, used on lots of high speeds stuff too, as well as other obvious things like government cars used in NI also.
There is a peculiar balancing technique, spin up 8 times bung weights on all is well, you can get a really really faint shimmy for the 1st hundred feet or so sometimes but nothing apart from that.
The recommend not for use below 50% profile but I've used it in 40's without problem.
tabetha