Edit/Change Custom Fields Lookup Type so i can preserve my data

Edit/Change Custom Fields Lookup Type so i can preserve my data

I need to add a new lookup field, but i've reached my max. I can't delete the existing fields that are set as lookup type because i will lose all my data. Is there any way to change the file type of one of my lookpup fields so it free's up a custom field and it saves my data?  Please help!