To be brutally honest, with the Hubble being so old, and by comparison to modern probe capabilities not very good, I would not wast the money...
I would put the cost of re-calibration - if it is even possible - towards a replacement.
The issue with tristimulus probes is the internal filters. If they have aged (which is very likely with an 8 year old Hubble) they would need replacing for any re-calibration to make sense.
I suggest you cut your losses, and get something new, and a lot better.
The Hubble was never that good with low-light measurements, and modern equivalent probes are a lot better.
I suggest the CR-100 and K10-A as the best alternatives.
Steve
Steve Shaw
Mob Boss at Light Illusion