is it possible to change the Specials field ?

Hello guys
I have a small thing i could use some help with

Currently we have put prices on every listing in our directory on https://familyfun.site . We used the special_offers field, and changed it to price ( it works as planned ) i imported prices for all of our listings.

Now we have fund out that it would be easier for the customer to choose a price in a dropdown instead of having to write their own price ( it would be much better for consistency ) Is there anyway to change the special_offers field to a dropdown instead of a single text field?
If not is there anyway to hide that field only on the add listing page ? the output works perfect for all our current listings, so the dropdown should only be active for new listings.

Do you have an idea how i can achieve this ?

thanks for the help