Updating a module field schema
As you mentioned, you already have textfield with 255 character length and also contains data. If you want reduce length of this field then you need to make sure that there is no data within this field having more than 100 character length. Because if this field contains any data more than 100 character length then you can't alter it in this way. My suggestion to you is first take backup of this table and empty this table. Then you would be able to change its length and then you can import this table data. If there is any data more than 100 character either it will stored in truncated way or give you any warnings for this. One thing you need to remember you need to do this offline, this question is specific to database and take care of every piece of data. There is possibility you might need to make some changes manually, so should take data base backup first.