Luckily, you can!  An article written in the Metro by Daniel Lublin, Toronto Employment Lawyer talks about the following types of changes that can be rejected:
Changed responsibilities.  If you have been hired for a defined role and that role is later changed without your consent, you may be entitled to leave your job and sue for damages as if you have been fired.
What about more work for the same pay?  In a recent case, an Ontario judge found that an employee was wrongfully dismissed when her job doubled but without any increase in pay.
Reduction in compensation.  You can reject pay cuts, however, the problem is that it is not clear how large the pay cut must be, since there are some cases where a five per cent reduction in overall pay could be refused but there are also cases where a 15 per cent decline was permitted.
Changed hours of work.  Your employer cannot change a specific shift or certain hours that have become a customary without your consent.
Harassment or discrimination.  If you objectively show that the workplace was intolerable and there was no other reasonable option but to immediately leave, you can leave work and then sue for damages.
Probation.  Probation period in generally cannot be imposed at their discretion, as a form of discipline.  Unless agreed to or used as last resort.
The full article can be found here, and for some more related articles please visit our web site.