Kim,
Given that you have a concern, it might be better to switch. Then you won't be thinking in the back of your mind (should I have switched?... am I increasing my chances?...).
If you didn't change and then you had a recurrence some day, you'd possibly start feeling guilty about not switching (whether it really made any difference or not).
I'd probably say the same thing if someone thought having their bedroom walls painted blue was the cause. If they have a worry about it, that isn't good. My belief is that the worry itself is a bad thing.
Your case is obviously and
clearly different because there was at least enough concern (in the medical community) about hair dye that it's been studied repeatedly as a possible carcinogen and certain chemicals are now not allowed to be used in dyes because of those concerns.
Point being that it isn't necessarily what the specific concern is as much as the fact that it's a concern. If it's easy to change and it removes the concern, why not just do it. <---(My humble opinion)
Compare the upside and downside of each option and go from there.
Worry = bad
Guilt = bad
Long term anger = bad
Bottling up negative feelings = bad
Happy = Good
An occasional cry if you're sad = Good
Confidence that you will are or will be cured (as long as you continue regular followup care) = GREAT
Mike