Park Hopper vs One Park cost... Please help!!

[Tomh]

When you do this - can you upgrade to park hopper for just that day (example day 3) and not the rest of your stay (6 day ticket)? If so, what’s the cost look like?

Park hopper is one price, for length of stay, regardless of whether you use it for one day, or for every day. So, this doesn't save you any money if you actually use the park hopper, but obviously, if you never end up upgrading to it, then you will have saved yourself some money.