The only airline I know of which has a good answer to that question is US Airways. When querying flight info in their system they always show the upflight number. Following is an example of today's flight US193. It is one flight number from FRA via CLT to MCO, but of course there is an aircraft change involved. You can easily tell that the plane from FRA was previously flying as US782 and the one from CLT was US605:

2US193«
FLIGHT INFORMATION
FLIGHT 193 - OPERATED BY US AIRWAYS MON 03APR 2006
***CHANGE OF AIRCRAFT***
STA SARRV SDEPT AGTE DGTE A/C EQP BCL DPZ UPFLT
FRA 1145A ¤677 330 782
CLT 315P 545P D11 B10 ¤622 757 12 605
MCO 718P ¤ 55

FLIGHT ACTIVITY
FRA/ OUT1144A .01E OFF1202P
CLT/ETA 330P .15L ETD 545P OT
MCO/ETA 718P OT

By now checking flight info e.g. for US782 one learns that this plane did not arrive from CLT but from PHL, and so on. Very interesting! I wish every airline gave out this information.

Edited to add: sorry that the above table is so hard to read. Multiple spaces seem to get automatically removed in posts.