This is my first post so I'll do an introduction, background and then get to it. I have a 35g tank which has been up and running well for 6+months. The whole set up was bought used so I couldn't do a proper cycle initially as it came with 3 fish. I re-used the gravel, filters, biowheel, and about 30% of the water from the previous owner. Ammonia 0, nitrite 0, nitrate around 20 consistently since day 1. I have had several plants, no co2, and plants are growing well. Last week added several more plants that are growing well too.
Fish: 1 common pleco 6", 2 giant danios, 2 platys, 3 female balloon mollies, 1 male black molly, 1 oto, 1 clown loach, 1 black fin catfish, 2 dwarf frogs. Have had several more black mollies but they all died fairly quickly, I think they were sickly when I got them. 1 giant danio recently committed suicide by jumping out through a 2"x6" opening in the lid and went unnoticed all day (my dogs aren't very observant) and 1 oto disappeared, never to been seen again.
Plants: 1 anubia, several vals, anacharis, and swords. No fake plants anymore.
Now the question:
HOB filter started making crazy noises recently so I got a Fluval U3 underwater filter. I did my homework and was confident in this purchase (I don't want this to be a filter pro's and con's discussion).
I have searched all over but can't seem to find a thread on any forum which talks about how to change filter types. Do I need to keep both going for a while or can I just let the biowheel float around for a few days or just turn off the old HOB and pack it away for emergencies?

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35g planted bowfront with driftwood
Fish: 1 common pleco, 1 bristlenose pleco, 1 black finned catfish, 2 giant danios, 2 balloon mollies, 1 black molly, 2 platys, 1 clown loach, and 1 siamese algae eater.
Plants: 2 amazon swords, 1 purple sword, 1 anubia, 2 bunches java ferns, 2 big bunches anacharis.

not familiar with your new filter but is there a compartment of some kind to put filter media in? I might just take the biowheel and filter cartridge from the HOB apart and put the filter media into the new filter.
The biowheels come apart easy, just pull the ends off and the corrogated filter part comes right out. Looks like an accordian.
I'd put whatever filter material the fluval needs in it, and then slip the biowheel media in with it. Even if you have to cut it up or something. Leave it in for a few weeks to seed the new filter and then take it out and discard it.

you say the oto dissapeared? bet I know why your HOB started acting up. I had an oto dissapear once, found it in my filter.

Just run both filters for a few weeks, then remove the old one. Or, put the old media in the new filter.

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"Being ignorant is not so much a Shame, as being unwilling to learn".-- Benjamin Franklin

Wow do I feel dumb. I never even thought about cutting up the biowheel and putting it in the new filter. And no oto in the filter or impeller, I did check that.

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35g planted bowfront with driftwood
Fish: 1 common pleco, 1 bristlenose pleco, 1 black finned catfish, 2 giant danios, 2 balloon mollies, 1 black molly, 2 platys, 1 clown loach, and 1 siamese algae eater.
Plants: 2 amazon swords, 1 purple sword, 1 anubia, 2 bunches java ferns, 2 big bunches anacharis.