I see what you mean. This wool pile is kind of installed in the frame itself. I always hate when window makers do this. I guess they never thought you'd ever need new wool pile? I don't get it.
Either way, it's not impossible. One trick I've used in the past is to run a straight edge razor along the wool pile as it sits in the channel. This will kind of bisect it into pieces and allow you to pull it out in strips. Then you'd get the new pile, push one half of the base into the channel, then use a firm, flat tool like a paint scraper to force the other half of the base in.
It's not elegant, and it will make the old sample hard to measure, but I think it will work for you.
Another idea would be to make a cut in the slot at the end of the channel where the wool pile sits and see if you can kinda pull it out that way. Then you will have an undamaged sample to measure.