During an upgrade and migration, tables are altered, renamed, deleted and recreated and this means that customisations made on column size/length may be overwritten, potentially causing data loss.
If after a migration, the column width is shorter than the text, the text will be lost.
Testing should be performed on migrations to ensure that this does not occur.
It is possible to compare column lengths from one version to another by installing the version being migrated to in a test environment, this would allow verification on any reduction in column widths from the live environment.
Different tables are affected in different ways during a migration and some customisations on column length may persist while others will not.