If I received a bonus for joining the Reserves, would I have to pay it back if I switch over to the Guard?

The commander has the option to release you from your obligation or not.
If I were to advise the commander, which way to go would depend on what the Guard offers in the new contract. If they give you a bonus, I would advise to rescind the Reserve bonus. Same with SLRP and MGIB (which you should get from the Guard as well).
The thing to look out here is to not obligate the Army to pay you twice, or create a pay problem with multiples of the same entitlement being paid on your behalf from different accounts.

In the event the Guard didn't pay a bonus, I would advise the commander to suspend the bonus, thus eliminating any future payments, and collect back any amount paid as a lump beyond the fulfilled commitment you have served.

Yes, if you were granted a conditional release from the USAR to the NG in the first place. It is not easy to "switch". Requires an approved DD368 by the USARC G1, not your unit commander. All he or she can do is recommend or endorse it. Your bonus was paid out by federal funds for a USAR enlistment contract. A conditional release terminates that contract, so any bonus paid to you must be paid back because you did not fulfil your contract wit the USAR. Bonuses are affiliated with unit and MOS in your enlistment contract. So it is not really "switching" at all. A conditional release terminates your enlistment contract with the condition that you enlist in another component on a new contract. USAR is a federal component, NG is state. Two separate entities.

I would say yes, a portion of that money paid out by the Reserves will be collected back, it depends on how long you served in the Reserves, how was the bonus paid out, lump, installments etc. But in truth if the UA or Finance does not initiate the paper work it could be a while or they could jump right on it.