Based on your question I will assume you fitted a full SP Engineering system, including the downpipes. In that case the downpipes are a different design to the OEM. If I recall correctly the mid pipe is removable and may, with some chopping, allow an OEM underslung unit to be fitted however I do not believe it is a simple slip on job. Also note the SP system was tuned for a bit more top end (shorter overall pipe length) so may present other performance than the original OEM system if the SP front pipes are combined with the OEM back box.
I had an SP stubby fitted to mine and replaced it for the same reason, just too anti-social even for me. My solution was to go for a Delkevic system. It fitted better than the SP Engineering and still gives a fruity exhaust note (important for me as I do lots of filtering around London) but is less raucous than the SP Stubby.
If it's of interest to you I have my old OEM box sat around in my garage which I'm happy to handover FOC but I won't be up for bothering with packaging, posting etc. FAIR WARNING; It's tatty (replaced after 25k hard miles), the end tip has come off and it was starting to 'chirp' indicating the inner matrix was developing a fault. You can collect for free if you want it but it will need some remedial cleaning up before fitting.
PM to arrange collection from Stroud, Gloucestershire. It's a fairly nice ride down the Fosseway through the Cotswolds so might be worth a half day ride out? IF I'm around you can also compare the note of the Delkevic to your current system.
PS - mine is also the superior Mk2 2011 Versys 650. Only nutters think the Mk1 is the fastest, and only accountants and spods prefer the Mk3.