As Kent mentioned, you can change your ticket, but it could be very expensive.
Although some people here have not seen anyone asked or stopped, I've seen people detained and refused entry. Usually because the immigration officer was suspicious of something.
Although it is very rare,but, if you are caught breaking immigration rules, it is up to the immigration officer to decide what to do. They could simply ask you to leave immediately or they could deport you. If deported, you could be banned from re-entering (and if it's a Schengen country that means all Schengen countries) for up to 10 years.
If sounds to me, and correct me if I'm wrong, you're trying to stay the maximum amount of time allowed, if not more,and perhaps find work to help pay your way? If this is so, there is a good chance you won't be caught. But there is also a small chance you will.
BTW...just because you get a "visa" for extended stay, whatever, it still doesn't guarantee you permission to enter or for the entire time. It is still up to the immigration officer. You didn't say how old you are, but if you're under 25 or 30, and you have a backpack, not a convertible bag, you will raise red flags. You may have to show a return ticket and financial resources to prove you can pay your way. Especially if you tell them you're "walking" from country to country. (ugh, they're bigger than you think.)
More than likely, you'll have no problems. But....there is always a small chance.
Since you admit you are a novice, I suggest you get a copy of "Europe through the Back Door" and read it. A lot of your questions will be answered.