At 26, the likelihood is that, even if this move doesn't work out and she comes back for a while, she will be looking to move out again soon, whether with another boyfriend, friend, house-share or on her own.
At 19, your other DD is still at the stage of finding her own feet as a new adult and not likely to be thinking of leaving home for a few years.
Even if your older DD's move proves unsuccessful and she moves straight back in and stays for a long time, she still doesn't automatically get the right to the bigger room. Whether or not you've lived in that house since she was born, she will have likely had the bigger/better room since birth - for 26 years - (and possibly also the smaller room as well as her own play room for 7 years, if you don't have any other children) and her sister has had a box room for 19 years, so she definitely deserves her turn now.
Whilst she was living at home, there would never have been an easy way to say "Right, time to swap now", but her choosing to move out, however successful or not it is, presents the perfect opportunity to slightly redress the balance.
Younger children often get a raw deal - they never have their parents completely to themselves and they invariably end up with hand-me-downs. With things like cots and high chairs, it doesn't really matter as there's a relatively short window when each child needs them before graduating to the next stage, but when it's actually a bedroom that they'll need permanently until they leave home, it's not so straightforward. In fact in some scenarios, where there's quite a gap, it makes send for the younger child to have the bigger room if they have loads of big, bulky toys and activities whereas a teenage sibling probably mainly only needs a small desk and a few compact electronic devices.
I can never understand the fashion for bedroom sizes in new-house design (not sure how old your house is, OP), whereby there will often be two similar-sized large bedrooms and then one or two much smaller ones. Surely, most families would find it much easier to have one bigger 'master' bedroom to be shared by the parents and then equal-sized smaller rooms for the children?