Here's a good thread on the topic.
http://www.homebrewtalk.com/f11/can-i-upgrade-typical-march-pump-hf-213577/315 HF impeller pt# 0809-0107-0200
You must have the high speed motor on your 809. This is from the HBT thread by the guy from March.
"There will be a label on the motor stating the rpm's of it. It says FASCO on it and It will either say 1300/1500/1750 for the low speed versions. And the "HS" versions will state either 3050 or 3500rpm. The "HS" versions are the ones you can change the impeller to the bigger one.
The "HS" motors are one of the following part numbers:
0809-0090-1000
0809-0123-1000
0809-0139-1000
0809-0196-1000
"