More tedious than painful. This also depends on the quality of mod records you have kept. The more mods you have, the more files you have to potentially merge with changed core files. Properly done mods using the override system save them from being overwritten, but if a core file that is overridden changes, you need to merge the mod changes with the version changes. Usually that's not difficult, just time-consuming, but sometimes the mod and the version have changed the same lines and you need to know what you are doing to get them working together. Checking all mods to see if there is a version specifically for ZC v1.3.8 is a good idea and can save you some work.