Question

he should change his subject change the voice​

Answer 1

His subject should be changed by him.

• The initially given sentence is in the active voice form, so we have to convert it into the passive voice form, in order to change the voice.
• As per the normal voice changing rules (active to passive), we interchanged the location of object and subject, with each other.
• And, other grammatical necessities are done (Adding 'be' after should, the third form of the main verb etc.) to properly transform the sentence.

Answer 2

Answer:

His subject should be changed by him.

Explanation:

It was a modal active verb case. Modal verbs are words such as can, could, may, might, would, should, etc. When we convert a modal active into passive, we use ‘be’ + Past Participle. The rest of the rules are the same.  

Thing to remember:

1. An interrogative active will remain interrogative in passive.