I will update this thread myself because it may be intersting to know for some people.

CCFJ1 wrote in a PM that the black cap is a straight replacement for the white cap, that I may need to do OS update, but he thinks I should be OK.

This is great news for me and it will be very interesting to see if there is a white cap or black cap laser inside my Mk4 unit. A white cap would explain why it refuses to read DVDs!