We are planning to visit Europe. The original plan was to visit two countries, let's say country A and country B. We were issued Schengen visa via country A since we intended to stay in that country for longer than country B. Our travel plans changed and now we want to spend fewer days in country A and more days in country B. We are now wondering if there will be issues during the visit?
In short, you risk running into problems at the border when you present the visa or are questioned by the border officer about your itinerary.
According to the Schengen visa code:
The Member State competent for examining and deciding on an application for a uniform visa shall be:
(a) the Member State whose territory constitutes the sole destination of the visit(s);
(b) if the visit includes more than one destination, the Member State whose territory constitutes the main destination of the visit(s) in terms of the length or purpose of stay; or
(c) if no main destination can be determined, the Member State whose external border the applicant intends to cross in order to enter the territory of the Member States.
You had originally followed the rules, in applying for a visa with the competent authority of your main destination. That is no longer true. Now, should you spend time equally in both countries A and B, and country A was your point of entry, then you'd be completely within the law.
Note: Emphasis added by me..
You may make minor changes to your itinerary after the application. Usually, moving the date of onward travel between two Schengen countries (or even varying the route) would be minor.
You may not misrepresent your itinerary during the visa application, especially if that would change the consulate which handles your application.
You should not give the appearance that your change was actually a misrepresentation. That could give problems with your next Schengen visa, and it might even get your entry refused.