While it can differ from case to case, changing column types should not generally be a problem with CMSB as long as the column type is suitable for the data in the field. For example, a field that contains only numbers (such as record num references) can pretty safely be changed from the default MEDIUMTEXT to an INT, which can improve performance for some lookups/joins. It is possible to do this within CMSB: when editing a field, under "Advanced Options" change the MySQL Column Type. You can also add an index to fields in this section by checking the "MySQL Indexing" option.
I would recommend making database backups before making changes like this, and if possible, testing them out in a staging environment.
Let me know if you have any other questions!