You don't need to un-register your module to introduce new code based custom fields. This is valid for both 4.0 and 4.1. I strongly suggest to first migrate your module to 4.1 since we will not be able to support problems with the 4.0 modules from now on and the ORM mapping is completly changed.
How this works in 4.1: Once you change dll-s of your module and restart sitefinity, upon intialization of your open access data provider, sitefintiy checks the build numbers of the assemblies that incude persistent classes mapped in your provider. If these build numbers have changed from the pervious initalization - it will test the database for consitency versus the new ORM model and if necessary upgrade - add columns, tables, indecies etc.. Hence if you add new fields and ORM mappings to your module you simply have to make a build of your module assembly with higher version than the previous ,copy the binaries in bin and restart sitefinity.
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