I am thinking of swapping the stems on my two bikes, but I am not sure whether this is possible/advisable.

when I got my Trek Soho six months ago I wasn't flexible at all and had a hard time reaching the handlebars. the LBS swapped the stem for a short one, and things were fine. nowadays and a couple thousand miles later, I find the handlebars a bit close, I think b/c I'm more flexible.

pretty sure I can take it back and they'll replace the short stem with the regular one. but I am thinking the short stem could be useful on my road bike, because...

...my Spec Roubaix is maybe a little large for me, so I strain a bit getting to the hoods/drops. I could take it in for a full fitting, but my guess is that after they tell me "you should have gotten a 52 instead of a 54" they'll suggest putting a shorter stem on it.

so I'm wondering, is there any reason I can't put the short stem from the Soho on the Roubaix, and vice versa? might make both look slightly like frankenbikes, but my visual appearance is nothing to write home about as it is :-)

Lastly, if this is not inadvisable is it also something I should attempt on my own vs having a bike mechanic tackle?

you can do this yourself, just make sure the compression nut is tightened down before you loosen the stem. I'm goingto assume you don't have a full carbon fork and steerer tube, in which case you would want a torque wrench to be on the safe side when you tighten the stems back down but otherwise this is a simple project that is easily reversible if it doesn't work for you.

The stems might not be compatible. Hard to say w/o measuring, but it's common for flat bar stems to work with a 25.4mm handlebar clamp diameter, whereas 26mm and 31.8mm diameters are common on road bikes.

Otherwise, the actual work involved is not so bad. It wouldn't hurt to have a supply of extra spacers on hand. Also read up on how to tension the top-cap.

This video might be helpful: https://bicycletutor.com/adjust-threadless-headset/

Thanks, will check it out!