Unfortunately no, you cannot change from single variant to multi-variant or vice versa on eBay. This is an eBay limitation
You cannot add variations/options if the product is listed already
You cannot remove variations/options if the product is listed already
You cannot rename variations/options if the product is listed already
You will get an eBay validation error such as "Invalid Multi-SKU item id supplied with variations."
In any of the cases above, the existing listing may be ended by eBay without warning. All listings that fit into the above scenario will need to be ended in eBay and the product re-listed as a new product which will have a new eBay item ID.
If you continue to see errors in eBayLink/Omnivore for these listings you will need to submit a ticket to have the item deleted from our database, when doing so please include the SKU and eBay item ID.
If you have a variation that you don't want to continue selling but want to keep the listing for other variations you can set the Quantity to 0 so that the product can't be purchased.