The answer is easy: Bill C‑14 already proposes an amendment to the Constitution. Clause 2 of the bill explicitly states how the text of the Constitution Act, 1867, will be worded going forward.
The only difference is that the procedure used for Bill C‑14 gives Parliament the right to do certain things, but not those for which it would need the agreement of seven provinces representing 50% of the population. What can't be done here, essentially, is to abolish the principle of proportional representation. There's a difference between abolishing it and slightly amending it, as is being done here for the territories. Can a bit more be done in other situations? That's somewhat the opinion that I've submitted to you.