I'm assuming that the size of the unit has been reduced pursuant to the change in the plan. Am I right?

One of the required clauses under the Act is that any reduction as to the size of the unit entitles you to a refund (based on the sixe of the reduction). Check your SPA to locate that clause.

However, it does not give you the right to repudiate (cancel) the SPA.

Did the developer give you a copy of the stamped SPA before they told of you of the intended change of the plan? In other words, do you have proof of the original plan?

If you have proof, then instruct your lawyer to write to the developer formally. Unless of course your lawyer is appointed by the developer (free legal fees?) 

In that case, you have to appoint your own lawyer.

After some bugging they might just decide to agree to abort the SPA and refund all monies paid by you. That is a long shot. They can just refund you based on the reduction.

I see. Then that clause is no longer applicable.

If both parties have executed the SPA it is deemed to be final. Stamping is just a mere formality. If you want to assert your right to the original plan (as at the date of execution), you need to engage another lawyer to send a letter of demand to the developer.

You need to instruct this current lawyer that you are not proceeding with the transaction due to the change in the plan.