Frames are, to a degree, just Erector Sets with welding, brazing or bonding holding the links together. So any tube is replaceable, sort of/kind of. Where you get tricky is with joints with more then two connecting tubes. Since most top tubes are joined at either end to only one tube (exceptions exist like Hellinec stay attachment) playing with one is pretty straight foreward.


First thing is to ditermine if the resulting frame geometry/fit will work for the next rider. Since lowering the top tube won't shorten the reach to the bars (by its self) this aspect should be considered, will a smaller frame also need a shorter reach to the bars? Finding the answer will potentially avoid a lot of work which might not work out after all. As mentioned above the rest of the frame's geometry also might not be ideal for a rider needing a smaller stand over.


Only then will the questions of the frame's construction be considered. Welded frames will have issues with localized distortion about the welded joint. If this area is maintained in the rebuilt frame then how the old weld area effects the new set up and joining is up to the skill of the "builder" to a great degree. Brazed frames have more leeway here as there's usually less local distortion/surface remnents of the OEM joint. Of course there's no rule that a modified frame has to have the same joinging methode as the OEM was done with.


To do this and retain the tube joints (with the head tube and seat tube) would result in an odd top tube path. First starting up high on the head tube (at the OEM location), then angling down to a stand over equating with what a 17.5" frame might have, then returning to the height that the seat tube attachment stub is at. Not sure if this really gains much in real life. There are reasons to retain the OEM joint and only replace a center section of a tube, but not when the tube's location is so radically different as the OP stated his goal was. I have done this a few times for various reasons but never to make a frame a different size or geometry.


Next up Are the seat stays. Where are they attached to the seat tube? Will the new lower size require to replace them too? If so then the job just got much larger in scope. Just removing a secion of the seat tube won't retain the stay attachment angles, like the top tube joints also will shift in there angles. Along the lines of a bigger job is the question of how well aligned is the OEM frame? Don't assume here as once you've changed the frame you own any issues whether they are of your doing or not. If you have to replace the seat stays then all tubes attaches to the Bb are free for alignment without effecting other tubes directly. If you don't understand this then I suggest you start your frame repair career on more simple jobs. If you have no ability to control this alignment you need to figure this out before commiting to the work.


I have lowered a frame only once. I was building a frame for a "friend" years ago (long before I became convinced of the need for business insurance) and he needed a very short reach yet a tall stack. In my youth I cracked the seat tube when brazing the stays onto it. I well overheated the joint, it's third heat cycle as I had done a special binder barrel after the top tube/seat tube was done. The crack was right below the seat lug bottom. I put this frame aside for the time, returned the deposit to my "friend" (and that was the best decision proven by hind sight later) and later returned to the cracked frame. I cut off the top tube by sectioning through the headtube and seat tube just below the lugs. A new top tube, both lugs and seat stays were installed. I ended up with a mid sized frame with a long reach.


The few top tube replacements I've done while maintaining the OEM geometry/fit have all been larger jobs then I first thought. Nothing went wrong just took a lot longer then I had estimated for. Repairing frames is a great way to learn a lot and to get stuck in job with expanding time/effort. There's a reason that experienced builders often only offer to repair their frames, not others' work.


Given the scope of the possible job and the likely lack of experience in the OP (and he wouldn't post here if he was sure of the job's issues) I suggest to pass. Andy