If you change a custom field's type does it wipe the data in that field for all contacts?

I don't think Marketo will even let you change the field type if it has data in it.

I think you are able to change it, but I've seen a warning that the data will be deleted. It's a little bit of work, but you could always export a list of the current field values with the user's email address, and then re-upload once you have changed the field over.

I actually decided to build a test custom integer field and loaded up a couple of my test lead with some numbers. I then changed the field to a string and it worked! No data was lost.

Dory, I'm still going to take your recommendation and export a list of the integer field and email address in case there is any lost. 

Thanks everyone.

Correct, only for certain type changes is the data deleted. In this case from integer to string it is not. 

As an aside, it would not be possible to change the type of a field if it was in use by another asset.