The GPL FAQ states that indeed, you can convert an LGPL binary to GPL
without violating the terms of the original LGPL license. But it doesn't
mean that you can link an LGPL library against GPL - you have to change
the license to GPL in this case

Further, since all the IMP.em source files would be available separately under LGPL (from an untainted copy of IMP from salilab.org), changing a particular copy of them (those distributed which are associated with any particular application) to GPL is a practically meaningless operation and not really worth worrying about.