Changing field type after data import......

I am creating a new form and decided to import the data from an excel spreadsheet since I had over 200 records. During the import the field mapping correctly shows the sheet and the data headers. I can choose the type of field. At first glance it makes sense to choose single line, but the field is for "manufacturer name". When the form is later used to create a new record, I would like the "manufacturer name" field to give the choice of selecting the "manufacturer name" from a list. I have another form which I have used to import all the manufacturer names for the sole purpose of providing name lookup for various forms.

So.... during import I can choose single line as the field type, but after import for future record creation I want the field to be a lookup pointing to the manufacturer list. I don't see the ability to change a single line to lookup.

How do I change the field type from single line to lookup? And if this can't be done, what suggestions are there for this?

btw I did ensure that all manufacturer names in the import list match the names in the lookup list.

Thanks in advance from a novice user....