Can I change to another reserved seat

If I have a reserved seat on the shinkansen, but the cabin is too warm , can i move to another reserved seat that is not occupied?

In theory you can until that seat is taken by its rightful occupant. The conductor may say something to you if he checks your ticket and you're not sitting in the correct seat.

generally speaking, you can't change the seat, until you get a permission by a train staff, 車掌.

No such situation.
Just extra tops slip off.

can i move to another reserved seat that is not occupied?

I have experienced several times when getting on a train only to find my seat was occupied. I just showed my tix, and normally the person would just move, and looked for an empty one. You surely could move to an unoccupied seat, just pray no one would tap your shoulder while you are half asleep.

You have to use the seat which you have reserved.

Regarding JR Shinkansen and Limited Express trains,
except when you change seat reservations before the scheduled departure time
(= the time for your train to depart from the station which you have reserved your seat from),
you cannot cancel the reserved seat and reserve another seat instead.

When you take a Shinkansen train from a station in the middle of the train's route, you probably do not have time to check the condition inside the cabin and change seat reservations while the Shinkansen train makes a stop. (That’s really a brief stop at most stations.)

If you newly reserve a seat in your Shinkansen train, which requires a Shinkansen surcharge (unless it is covered by your JR pass),
logically you can abandon your right for the seat which you have reserved,
but you are excepted not to do so, because that may deprive another passenger of a chance to use that seat.

[When you hold a valid Shinkansen / Ltd. Express reserved-seat ticket]

Before the scheduled departure time, if there is a vacant seat which can be reserved, you can once (but only once) replace your Shinkansen reserved-seat ticket by a new one and change seat reservations, with no handling fee required.

This rule does not apply when the reserved-seat ticket which you hold has been issued for a replacement (mentioned in the preceding paragraph or the following section), in which case your cancellation requires a handling fee.

You cannot replace your reserved-seat ticket by a non-reservation ticket.

[When you hold a valid Shinkansen / Ltd. Express non-reservation ticket]

While your non-reservation ticket is valid, if there is a vacant seat which can be reserved, you can pay for the gap between the Shinkansen / Limited Express surcharges and replace the non-reservation ticket by a reserved-seat ticket, with no handling fee required.

Note
: In some Shinkansen and Limited Express trains, every seat needs reservation.
: When your JR pass covers a Shinkansen / Limited Express surcharge for your ride without seat reservation, you don’t get issued a Shinkansen / Limited Express ticket.

Just for clarification:
Let me add "first" after "your right for the seat which you have reserved" in my previous message.