This articles provides detailed information on the supported data/dictionary type changes that users/customers can perform on their instances.
Changing data types
Data/dictionary type changes are not recommended but if a you need to perform this action it is important that there is a clear understanding of what can and cannot be supported. By default, ServiceNow supports data type changes where the alteration does not constitute an "online ALTER" where the data type from the database is changed.
A data type change where a string is changed to an integer is not supported as both of these data types from the front end map to entirely different data types in the backend and will most likely result in an online alter. Please note that performing such changes where data is present can result in data loss when making unsupported changes. If a data type change is absolutely necessary ServiceNow recommends creating an entirely new field of the type that is needed.
Decreasing max length of a field is highly not recommended as this can result in data loss
A data type change where a user increases the max length of a field from 300 to 4000 is a supported change since we would still map to the same field type from the database backend. The rule of thumb is that any data type change where a change from the database backend is not warranted and an online alter is not required is a supported change.
Documentation topic: Dictionary entry data types