Unfortunately no, you cannot change from single variant to multi-variant or vice versa on eBay.
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 needs to be terminated, deleted from our database and the product re-listed as a new product.
You will need to submit a ticket to have the item deleted, 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.