You're confusing "fares" and "fare buckets".

You've booked an fare who's code likely starts with R. Something like RLX14ABC, which is a discount Premium Economy fare. This fare allows you to book on a flight that has either "R" or "B" class available - but at least one of the legs must be in R. This allows you to sit in Premium Economy (R) on flights that have it, but in standard economy (B) in other flights.

When you try and change the flight and force it to book in "B" (without at least one leg in R) then the above fare is no longer valid, and it'll instead pick something likely starting with B, which is a near full price economy fare, and will be significantly more expensive than the R* fare you've already got.

As has already been said, changing the ticket today will (probably) re-price the entire ticket as if you'd bought it today. If you wait until after you've flown the outbound, it will (probably) re-price based on the prices when you originally booked the ticket, but it will be based on the availability at the time you're making the change which could well be lower than today. ie, it's possible it's better to change today, but also possible it's better to wait and do it after the first leg - there's no way to know which will be better.

The reason I've put '(probably)' there is that it technically depends on the ticket rules, and the comments you've made about no refund when changing to lower fares makes me think that you're not on a normal/publicly available ticket/fare, as generally you would get a refund in a situation like this, at least for tickets starting in the US.