You can switch whenever you want. If you switch mid-month then you'll be charged pro rata. No minimum requirements, you can switch to anchor level whenever you're ready. Ebay leave it to sellers to work out when it's financially beneficial to change shop level, they won't advise when to make the change.
Ebay's only aim is to make as much money as possible from business sellers with as little effort as possible so they certainly won't discourage you from going for the most expensive option. In truth, there are no other worthwhile benefits than a greater number of inclusive listings so just base it on that.
