It's a bit tricky because of the whole r/t ticket, but I'm pretty sure that leaving the outbound alone is okay and will not trigger a re-validation. I would definitely do the math first to make sure you are quoted the right amount.

Assuming the outbound is allowed to stay in tact, for a return date of 04-Jul-19, and assuming the original inbound was ticketed recently (to a tariff of LHX91NCT) I see a fare difference of

$691 to Premium Economy (N) -- LH Y connecting to LH N
$1,216 to Premium Economy (E) -- LY Y connecting to LH E (GPU eligible)

with a change fee of USD 300.00 (which could be waived if you ask nicely).