I think if you change any aspect of an L fare you are going to be dinged the change fee, whether it is one leg to H or two.

As far as the fare difference goes, sure, that will depend on what exactly is being changed. In the OP's case I think the cheapest he can do is move from L/L to H/Q, since L is not available. He could also go to H/H

I say I think because I have no experience making a one-leg booking class change when the current booking class is no longer available (or couldn't be purchased according to its rules). Maybe he is allowed to change it to H/L.

The real question is, is the upfare to H worth it at $1000 or so to get the upgrade? We'd all say yes, but that's because we're crazy!