Prior to the fellowship split in 1961, were teachers and perhaps pastors from both WELS and LCMS, for a lack of a better term, interchangeable? In other words, could an LCMS pastor or teacher accept a call to the WELS and vice versa? I'm almost 55 and grew up in the LCMS (later joining a WELS church in 1992 and been a member ever since). Recently, I learned that a teacher at the LCMS grade school I attended passed away. So would I be correct to conclude without knowing his complete work history that he may have gone back and forth between the WELS and LCMS prior to the 1961 split? So what happened to teachers and pastors that had LCMS upbringing and education were serving in the WELS right after the 1961 split? Were they grandfathered in or were they required to take some instructions at the various WELS colleges in order to keep their job? Now of course things are different so I don't know how the LCMS handles such things but I would imagine that an LCMS pastor that desires to be called to a WELS church would, in addition to becoming a member, be required to have extensive training at a WELS college and/or seminary first regardless of previous education in the LCMS system? Any help or correction would be appreciated since I'm very curious on both the current and historical practices.
Your understanding of the “interchangeability” of WELS and LCMS pastors—and the teacher you had—prior to 1961 is correct. When, prior to 1961, WELS and LCMS pastors accepted to calls to congregations beyond their synod, they became rostered clergy of that synod.
You are also correct in noting the importance and necessity of a colloquy process for an LCMS pastor who today desires to serve in WELS.
With your background, you may be interested in reading A Tale of Two Synods: Events That Led to the Split Between Wisconsin and Missouri. It is available from Northwestern Publishing House.