If you mean a constant TAS of 250 kts then the IAS will decrease as you climb.
If we go back to the dynamic pressure equation we can see why.
Dynamic pressure = 1/2 Rho Vsquared
Rho is the air density and V is the TAS.
So in a constant 250 Kt TAS climb dynamic pressure = 1/2 Rho 250squared.
As altitude increases, the static pressure decreases. This causes the air to expand, so its density, Rho decreases.
If we hold TAS constant while we climb, then the 250squared stays constant while the Rho decreases. So the dynamic pressure must decrease.
The ASI simply measures dynamic pressure, so its indication will decrease.
So if we climb at a constant 250 kts TAS the indicated airspeed will decrease.