Significant design changes were only made on the early versions of the Pi, this doesn't include the Pi 2 Model B.
However for the Pi 2 Model B there was a total of 2 revisions to the PCB. However, I can't seem to find detail on what some of those revisions were.
To check what revision you have open a terminal window and run:
You should get an output that looks like the following:
Processor : ARMv6-compatible processor rev 7 (v6l)
BogoMIPS : 847.05
Features : swp half thumb fastmult vfp edsp java tls
CPU implementer : 0x41
CPU architecture: 7
CPU variant : 0x0
CPU part : 0xb76
CPU revision : 7
Hardware : BCM2708
Revision : 0002
Serial : 000000000abc0ab1
The bit you are interested in is the Revision. However the fact it says
0002, does not actually mean its a Pi 2 Model B revision 2 board.
To break it down for you:
a01040 - Rev 1.0 board manufactured by Sony
a01041 - Rev 1.1 board manufactured by Sony (in the UK)
a21041 - Rev 1.1 Board manufactured by Embest (in China)
a22042 - Rev 1.2 Board manufactured by Embest (in China)
Interestingly on the Hardware History Wiki Page it notes that the 1.2 board features the BCM2837, which is the Broadcom chip used in the Pi 3. The underlying architecture between BCM2837 and BCM2836 as used in earlier revisions of the Pi 2 is the same. The key difference is the ARMv7 quad core cluster was replaced with a quad-core ARM Cortex A53 (ARMv8) cluster.
There does not seem to have been any revisions to the Pi 3 Model B. If some unexpected fault is discovered, I assume a new revision would be created. I'm not sure that any significant hardware changes are going to be made to it.
Also a "Pi 4" is to not going be released in 2017 according to Eben Upton the Pi's co-creator. And also on the topic of the rapid realease of the Pi 3 after the Pi 2 he said.
We're kind of at the end of that particular roadmap. I would expect a longer pause, a couple of years at least, before any kind of major bump to the platform
So don't hold out for a Pi 4.