1) Install RELENG_14 on a VM and put a kernel from RELENG_15 into it.
2) Build and install both `sbin/ipfw15` and `sbin/ipfw` from sources
3) Try to add/remove rules, etc
Running the patched ipfw on RELENG_14 will produce:
```
r14# ipfw show
WARNING! KBI incompatibility for ipfw is detected, trying to run /sbin/ipfw15.
01000 12844 2993997 allow ip from any to any
65535 0 0 count ip from any to any not // orphaned dynamic states counter
65535 17 3208 deny ip from any to any
r14#
```