I'm using an open source library under GPL in a closed source project. If the libraries license changes from GPL to LGPL, does it mean the previous code (written when the library was GPL) should be made open source?
The GPL is already an open source license.
But if you relicense a project (and a license change from GPL to LGPL needs the permission of all contributors, I assume that this is given) does not change the license on the old code. So the previous code is still available under the GPL, nothing changes in that regard.