The usual rule is that, both for a single-entry visa, and for the first visit on a multiple-entry visa, you should do what you had described in your application (on further visits on a multiple-entry visa you can do whatever you like as long as you stay within the allowed limits).
While minor changes in your itinerary (a day here, a day there) are not a problem, major changes, especially those that involve a change in the reason for the trip or the main destination, can be an issue.
Remember that the issuing country does not need to be the country where you enter the Schengen Area. It must be your main destination (for that first visit).
Whether this will actually be noticed or not is difficult to predict. Some countries are extremely strict and will get notifications from hotels if your bookings are cancelled for instance, and will send you e-mails menacing you of cancelling your visa if you don't stick to your plan (obviously not the case here, but that illustrates how far it can go). Others will only check what happened on your next application. Others will not care whatsoever.
Likewise, border officials may ask a lot of questions or very few. If they do ask for an itinerary and for any evidence of it (bookings, etc.) they are likely to notice the discrepancy and raise an issue. They are probably less strict with holders of multiple-entry visas (especially if you have a longer history of visits to the Schengen Area before that), but still, the first visit counts. Again, it may vary a lot from country to country, and it helps if your entry point is the issuing country, but it's by far not a guarantee.
Your first proposed itinerary changes the main destination nearly without a doubt, and that can definitely be an issue on a first visit. If it also changes the purpose of the trip (e.g. it was originally a business trip and now you are going for tourism or to visit relatives), that can be even more of a problem.
Your second proposed itinerary is a bit more on the edge, as you have equal time in all countries, so the first country is usually considered the main destination (unless there is a reason to consider another country as the main destination), but you may get into considerations of how time in each country is counted (days or nights?).
I would strongly recommend you stick to your original plan as much as possible. If not, you should probably ask the issuing consulate/embassy, and you may need to apply for a new visa to be on the safe side.
