Good reason to retain my 40 year old Crime of the Century UHQR?

It's your call. If you haven't listened to it since you can't remember...sell it! Then, buy something you want now.

Depends on  your wants entirely.
If it is condition you say it is possible a $500 album.
IF you can find someone to buy it who wants it badly enough.
Always only worth what someone will pay when you get down to it.

Good luck in your decision.

Me?
I would not be thinking of selling but of playing it!

I have the standard MOFi and would never consider selling it but it's not worth anything near yours.I'm always trying new upgrades on my turntable and system and my records sound better than played a few years ago so I keep my LP's as they could sound better in the future.

Playpen
I also have the "standard" MOFI release which is more like a $25 album but the SQ is off the charts.
This is still one of my goto albums for testing changes made in my system.

I am not sure how much better the UHQR version can get tbh, certainly not 20 times better.

Good luck to the OP whatever he decides as it is a very nice and valuable record indeed!

Seems like an easy decision if you're not a collector, but an "enjoyer". If you don't listen to it (much), then you can always get another version for a lot less and casual listening. The going rate could buy a lot of software you'll enjoy.

Don't know what country you're in, but a quick check on Discogs shows 4 for sale, the only US seller wants $750 (NM). You could always offer it there for top dollar, and if somebody bites congratulations!

I have an original Mofi and a 1/2 speed Canadian Audiophile  of COTC and both sound superb. 
Sell it,buy a used mint MOFI copy and spend the 450 left on music or whatever. You will still have an excellent recording of COTC to turn too.