I also switched from an HOB
to a canister. I changed because I wanted better filtration, but even more because of the fact my HOB
was so noisy.
I left them both running together for about 2 weeks, and during that time I slowly moved every single piece of filter media from my HOB
into the bio-media area of my new Fluval 405 to help seed the ceramic media. Once the HOB
was empty and everything was moved over, I let it run a day and took it down. There was no movement in any of the parameters.
To this day I still have filter pads from the old HOB
jammed in my canister with the ceramics...I don't throw any bacteria away unless it's absolutely necessary. It also helps to have it in there in case I need to break out the HOB
to set up a QT, or if a friend needs some seeded media