What are the legal requirements to change your hours to help another work collegue (which are not your contracted hours?

Hi there I’m having some trouble at work and have been moved out of my department. My contracted hours were 10am till 2pm, but around a year and a half ago I changed my hours to help out my work collegue, I spoke with my manager and began working 8am untill 12pm. I didn’t sign any documents, and now 18months down the line I wanted to go back to my contracted hours, I have been granted this but have been moved out of the department I work in. Can anyone help me here as I don’t know much about legal requirements here. I am currently waiting on a member of unision to contact me.
What are the legal requirements to change your hours to help another work collegue (which are not your contracted hours?
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