... I find your posts very helpful and eye opening. They are thought provoking and greatly appreciated.
Perhaps the main thought this should provoke is the need to periodically review your policy limits.
Hagerty uses agreed value and offers an online valuation tool if you need help pondering what value to request. Increasing the value with Hagerty couldn't be easier - you typically don't even have to talk to someone. On their website you can increase the agreed value and you'll get a reply from an underwriter in a day or two. Takes about 2 minutes. I have never had a request increase turned down - they want you to be fully insured.
I have even heard of a case where they contacted an owner with concerns that he was underinsured (a big $$$ car - don't wait for their call for what we drive).
The increase (at least with Hagerty) does not have to be at renewal -- any day is fine and they will just bill you for whatever premium increase is needed for the period until your next renewal.
When I first get a car I usually just insure it for what I paid, then increase as major parts go into it. Eventually I set it at the market level once the car condition has reached the level I was going for.