Legally changing your name is not the same as assuming.
In Ontario, if you assume your husbands name your documents (i.e., license, passport etc) can be his last name or a hyphenated version but your birth certificate remains the same. If you wish to use your legal name again, you simply revert back by presenting your birth certificate as proof of your legal name.
If you legally change your name you actually change your birth certificate which is why it's so hard to change it back.
Here's the link to the government site..
CHANGING YOUR NAME DUE TO A RELATIONSHIP & CHANGING IT BACK - Health social services - Gateway Individuals - Government of Ontario (Canada)