If you keep your IAS constant while you are climbing your TAS will increase and you Mach too. And opposite when your are descending.
If you keep your TAS constant while you are climbing your IAS will decrease and your Mach will increase. And opposite when your are descending.
If you keep your Mach constant while you are climbing your IAS will decrease and your TAS will decrease. And opposite when your are descending.
If you climb with a constand IAS you will be limited by the MMO. The same when you are desending, if you keep a constand Mach you will be limited by you VMO.
Usualy you climb with constand IAS up to 25.000 feet and then you will keep a constand Mach. And opposite when your are descending.