For a detailed discussion of what is or isn't allowed, you can refer to Should my first trip be to the country which issued my Schengen Visa? or Can I change my route and hotel reservation after getting a Schengen visa? and all the linked questions.
In your case, if your visa has enough validity (i.e. it must be valid for the whole period you would be in the Schengen area and authorise at least 15 days of stay, ideally a few more) and the purpose of the trip is still the same, it should not be problem. Belgium is still your main destination and you will still enter and leave through the same countries so no reason the change should be an issue either during the trip or afterwards.
If your visa cannot cover the whole journey (i.e. it does not allow 15 days of stay or it would expire before the end of your trip), then you need another one. There is no simple established procedure to get an extension, you will need to get in touch with the consulate, possibly start a new application from scratch.