steel shingles over tin

In April we had a new Futureroof Slate-look roof installed. One of our bay windows has had a painted tin roof since 1981. We decided to match the bay roof with the rest of the house. We were reluctant to remove the tin roof as it very solid and we didn't want to get into a major rebuilding project. The installers attached the Futureroof steel directly to the tin. It appears to be fine, but I was wondering if this will present any problems down the road. Thanks in advance!