Hi Jos,
Not Possible to Change Any thing in UDF after Adding.
Only you can Increase Length of UDF in case you have Created Alphanumeric UDF.
In Addition, Please check below Link.
http://www.pioneerb1.com/wp-content/uploads/2012/06/How-to-Create-User-Defined-Fields-and-Tables.pdf
Hope this helps 
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Regards::::
Atul Chakraborty