When my company set up Marketo originally they decided to create a string Field that clearly should have been a date Field. We'd like to convert the Field in question to a date field but are worried about the ramifications of doing that. Will the existing be deleted if we do that? Are there any other things we should be considering?
Changing the field type from string to date will destroy the data in that field. You could export a list first though, and then just re-import to the field once you've changed it, but you may need to change the format of the value, depending on what the date field will accept and how the string is formatted now.
In order to change the field type you'd need to remove it from all assets currently referencing it as well, so if you have a lot it might be easier to create a new field and start using that one instead.
The conversion will clear out all your data in that field. You'll want to export everything in that column to back it up and then reimport it once you're down with conversion. This might b more complicated than it seems at first blush though, becuase of potential formatting and timezone issues. I would strongly consider just making a new field, migrating your data to it and then hiding the old one.
Full steps would be:
1) Create the new field with the correct field type
2) Export lead and old field's data for all leads
3) Migrate all the data from the old field to the new field in excel, checking for formatting and accuracy
4) Upload the updated csv file as a list in Marketo, allowing all the data to be updated
5) Delete the old field Hide the old field
EDIT: As pointed out by John Clark, it's not possible to truly delete fields.
It's actually not possible to delete a field in Marketo. You'd need to hide it instead.
I thought you could,assuming it was not used in any asset and the field was NULL.