you can do it to save the strain.. be ready for a long wait while it reverts to veg... but you should still be able to do it.. hell you could harvest all but the bottom of the plant.. trim the rootball back and put the plant back into veg and it should take back off too..in time.
I am experimenting with taking flowering clones too.. I just did some at 3 weeks.. next time will be 4.. to test rooting times..but I personally wouldn't do it later than that as usually by 4 weeks I have some pretty good bud sites, BUT in your case it's different - to keep the genetics. take from lower branches if you have them (meaning if you didn't already lollipop them)- good luck.