Apparently the Convert To Sheet Metal command has always had a land mine hidden in it - using the command will wreck any mates associated with the part in an assembly!  Some research shows that the bug was discussed in the user forums at least as far back as 2010, but Solidworks apparently can't be bothered replying to the original query, fixing the bug or at least sticking a warning in the command.  Or even sticking a warning in their virtually worthless online Help files. 

So I guess the real question is, what the hell does it take to get this company to Fix Their Product?