Interesting idea. Structurally, I bet that very little modification would be needed for the parts to all fit together. The wing ribs are identical, the spar components are in the same place... If the spar bar thickness at the fuselage is the same or close, then it should be doable.
Might need a bigger horizontal stab...
Indeed. With more wing area, you want more horizontal stab area.
Keeping in the spirit of this post, you could probably install an RV-10 horizontal stab onto an RV-14, with only minor modification if any. (They both share the general shape and structural layout of the RV-9 horizontal stab, all with identical ribs and many other components, but the -10 stab has more span i.e. more ribs and a longer spar).
...the extra span would take some strength out of it, making it a non aerobatic platform...
Doing some back-of-the-envelope math... I don't know the spanwise lift distribution (you could assume Schrenk's approximation...) but they're probably the same shape. If the RV-10 span is 32 feet and the RV-14 is 27 feet and the fuselage width is approximately four feet, then each RV-10 wing is 14 feet and each RV-14 wing is 11.5 feet... so the bending moment at a given load would be ~21.7% higher with an RV-10 wing versus an RV-14 wing... so limit load goes down from 6g to ~4.93g if you don't want to over-stress the RV-14 fuselage.
(The wings themselves could probably take the load. The RV-10's 2700 pounds times 4.4g is actually about 4% more load than the RV-14's 1900 aerobatic pounds times 6g).
Climb performance would probably be even better, due to the higher wingspan lowering the induced drag. (So, like with the RV-9 versus the RV-7, this theoretical airplane would need less horsepower than an RV-14 to climb). The impact of this is greater at slow speeds and smaller at high speeds, so cruise speed would probably go down due to the added surface area (even with the same engine as an RV-14).
The stall speed would decrease a little bit (especially given the RV-10's bigger flaps) so you'd probably end up needing somewhere between 18% and 25% less runway to land than an RV-14.