The Packers have somewhat a glut of OL. OL are always in demand, and GB could be in a position of having to release a guy they would normally keep.
Tretter may not have a future in GB, with Linsley the starter at center. He seems a bit light for guard, so could be behind both Taylor and Rotheram as possible replacements at guard if Sitton or Lang leaves. The two rookies diminish his value as a backup tackle this year.
Rotheram is working at center this off season. If he can be an adequate backup for Linsley, trading Tretter at the end of training camp could be a possibility.
Another possibility for a trade is Barclay. He had a miserable 2015 coming back from the ACL surgery, but he has significant starting experience during which he was tolerable. If he can bounce back this year, he could be worth a 7th round pick to someone.
They could trade both and still have 9 OL.