If I upgrade one of my old PC's CPUs that is the same CPU only clocked higher could I use the same heatsink and fan. This is an old pre-built I am upgrading because well the CPUs are cheap and it's fun tinkering around with old stuff :)
Yes. A motherboard is built around a "CPU Socket" such as 775, 1366, 1155,1150, 2011, etc. A heatsink is built for the socket, not a CPU.
However, a heatsink will only cope within a certain Thermal Design Power (TDP). For example a stock Intel heatsink many only cope with 60w TDP, and the processor its cooling is a 54w TDP. If you overclock it or put a hotter CPU under the hood, the heatsink will simply struggle to dissipate the heat.
So in reality, if your PC isn't hitting any notably high temperatures, your cooler can likely cope with something with a higher TDP.
I'm just upgrading an old Pentium 4 2.80 GHz to a P4 3.02 GHz. I think it should be fine.
Shouldn't be a problem.