There should be no problems. Such minor changes of plans are completely allowed, at least as long as the main reason for the trip is still the same that you applied for originally.
As a practical matter, there will be no way for border guards in Budapest to even know that your plans have changed since the visa application. If you had planned to enter via Hungary from the beginning, you would still have filed your application at the Austrian consulate (since Austria is your main destination), and you would have gotten a visa that's indistinguishable from the one you actually have.
At the border in Budapest you may be expected to show that you have credible plans for continuing towards Austria -- but if you can explain without hesitation which train you're going to take (and either have a ticket for it or have researched where to buy one and how much it will cost), where you will stay in Austria, etc. that ought to suffice.