Is a diploma riskier than a degree for the work permit afterwards?

There is no rule about academic downgrade. No provision mentions progression, and nothing in the system compares your next credential against your last one.
What an officer decides is whether the plan makes sense for you and whether you would leave at the end of it. Going from a master's to a diploma raises the question of why, and that answer has to be in the application rather than in your head. It is a writing problem, not a disqualification.
The real difference is somewhere else entirely
The work permit after graduation has a field of study requirement, and who it applies to is published. It applies to anyone who "graduated from a program that does not result in a bachelor's, master's, or doctoral degree."
So a diploma has to be in an eligible field. A degree never has to be. With the degree, the question simply never arises.
The second difference is length
The permit runs from a minimum of 8 months to a maximum of 3 years. For a programme of 8 months to under 2 years, the permit should match the length of the programme. For a programme of 2 years or longer, it should be 3 years.
There is also a measure for degree-granting master's programmes shorter than 2 years, where a 3-year permit can be issued even though the programme is short. Graduate certificates and graduate diplomas are excluded from it.
That is worth putting beside the tuition. A one-year diploma points toward roughly a one-year permit. A sixteen-month master's can point toward three.
Whichever you choose, there is a deadline to apply for that permit once you finish, and missing it is a problem of its own.
Source
Post-graduation work permit, IRCC program delivery instructions, modified 11 December 2025.
If you are weighing specific programmes, the institution and the exact programme name are what decide both of these. Post them, and we will go through what each one actually leads to.