Hello there. When they get older and have difficulty climbing it does help to have them in a larger, one level area and it allows them to keep being active as long as possible. Quite understand your concern about a cage change, but if done carefully it will be much appreciated by her I am sure. I remember another member on here doing it with much anxiety and her hamster loved it and was much happier.
Not sure what your unused one level housing is but I'd say 80 x 50cm to 100 x 50cm low level cage or perspex tank style would be ideal.
Within that you wouldn't want anything she could climb and fall off - but they still need some overhead cover, so hanging a large hammock from the top can achieve that (she shouldn't be able to get up there to climb into it but it will make her feel more secure underneath.
When our Syrian got old and had everything on one level I added a couple more large floor toys to make it more interesting - a large cork log - which was a tunnel but also a low level interesting texture for climbing over. And a large rat tube tied to one end of the cage just above the substrate, plus a small hammock in a corner.
I don't normally like fabric hammocks in cages but our Syrian was so old and laid back he was beyond chewing hammocks
The way to do it, I would suggest - is fill the new cage with substrate and 2 or 3 toys - a couple from her current cage and maybe a new one that will stay in there - and let her play in it for out of cage time for a short time. If she shows signs of stress or wanting to come out then put her back in her old cage. Then do the same thing next day. Second time it will be more familiar and she should want to play in there a bit longer - you could hide the odd treat for her to find. Then back to her old cage for the night.
On the third day you could move her - take everything out of her current cages without cleaning anything at all - resist the temptation! Take all her old substrate and spread it out on top of the new substrate in the new cage. Then set up the new cage with her old house, wheel, toys etc, in more or less the same layout as before - as similar as possible - eg wheel right hand side of house, food bowl on house, tunnel near house etc. Obviously it won't be exactly the same if things are spread out between cages but as similar as possible.
You can use some of the connecting tubes from the old cage in short runs as floor toys so they will be familiar smelling as well. Move her nest and hoard over as well.
Then scatter some food in the cage and put out new nesting material - she may decide to abandon the old nest and build a new one or she may just rearrange the old one and nest in it again. Try to put her hoard back where it was (eg under the nest).
This way she should settle quite quickly - that's what I've found doing it this way.
Our last Syrian even when he had very stiff back legs - used to whizz around the cage quite fast when it was all on one level and still used his wheel for a few minutes at a time. With the wheel try and get it so it's only fractionally above the substrate so no chance of tripping when getting in and out of it.