I have two Seagate 5400.6 HDD.

One seagate 5400.6 HDD
ST9500325AS
PN : 9HH134-231
FW : 001SDM1

HEAD : WY4-T / TICB-B

Another seagate 5400.6 HDD
ST9500325AS
PN : 9HH134-231
FW : 001SDM1

PN,FW are all same. but head part NO. WY4-P / TICB-P

I have tried to replace each normal Head but failed.
I would like to know why should not compatible with the same PN and FW.

Please answer my question.

The part numbers may be the same, but the topography of the sliders may be in fact different. You have to compare them under magnification to get a true match.

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http://www.datasaversllc.com

then, How to interconvert each other?

No feasible method to convert between them, or at least not in a physical manner. It is just like any other part applications in other industries.
The part either is compatible or not.

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Hard Disk Drive (HDD), Solid State Drive (SSD, SATA, NVMe, etc), USB Flash Drive and RAID Data Recovery Specialist in Massachusetts