Hi Jeroen,
Good with the points 1&2, so I think this shouldn't be a huge problem to change the data type in the BO.
Please go ahead and change it but make sure all the tabs/UI's and scripts have to refreshed/Update Metadata and activate. This is a time consuming task but no other go to ensure the robustness and flawless functionality, please do it.
There is no other data type which can accommodate more than 255 Chars 
.
Regards
Hanu K
