Conversion of Alif Maksurah

[Tanweer]

As-salamu Alaikum.

If a pronoun is joined to a word ending with an Alif Maksurah, then the Alif Maksurah converts into a normal Ya or a normal Alif.

Example: (Conversion to a normal Ya)

إلَى + هَا = إلَيْها

Example: (Conversion to a normal Alif)

مَعْنَى + هَا = مَعْنَاهَا

My question is: When does it convert to a Ya and when to an Alif ?