When I added my bikes to the home insurance, the chap on the phone basically said to insure it to the value that it would cost to replace. So if, for example I’d paid 3k for a bike in the sale which should have been 5k, it gets insured for the full RRP.
Same with upgrades. I’d put new forks on so he said to add the value of purchasing the fork new to the insured value of the bike.
So my theoretical 5k bike to which I’d added a theoretical £1k fork makes the insured value 6k. That’s the replacement cost, even though the actual cost would have been less than that.
The following year at renewal time, I guess you’d need to bump the value up a bit to compensate for the ever increasing prices/ decreasing spec of newer bikes.
If you had a trail bike with XT and 36s, you’d need to see what the same spec trail bike would cost and insure your bike to that value. If you don’t, you’d probably end up out of pocket if it was stolen.