Q:

What would be the problems to change Tourism UK visa to EEA family permit?

I have a relationship (2 years and 7 months/NotMarried) with an European (I’m from South America, then I need a visa only to enter to UK. We live together in Europe since one year (1 year and 5 months more or less). 
He will work for 6 months in UK, but there exists a possibility that the contract will be extended for one year (or more), but it is only a possibility. I want to go with him no matter if the contract is extended or not. If they give me the visa I will live with him in UK. 
In the case that the contract won't be extended, we will return to Europe, then that's why I don't want to search for a job in UK right now (and I don't think in short time I can find a job). But in the case that the contract will be extended, I want to stay with him, but I also want to work in the meantime (qualified work). 
As I'm not sure if they accept my application as EEA Family Permit, I want to know if for example I go with tourism visa and if his contract will be extended, can I change my tourist visa to a EEA family permit?
Due to I will be living with him for almost 2 years at that time, maybe then I have more chance to apply to the EEA family permit and also by that time we maybe get married. Because If I apply directly to the EEA family permit maybe they denied my visa and I heard that it would be a problem if you apply again (because in your historial exists a denied visa).
I want to know what happened if I ask for the tourism visa and then I apply for the EEA family permit, Do I have to leave UK if I want to apply to the EEA family permit after the tourism visa? What would be the consequences? 

A:

At first it doesn't look good. According to gov.uk,

You must be outside the UK to apply for an EEA family permit.

On the other hand, the family permit is just for entering the UK, not for remaining there once you're already in (your right to remain would derive directly from your boyfriend's freedom-of-movement rights).
If you're already inside the UK, you can apply directly for a residence card rather than a family permit.
However, your plan to arrive on a "tourism" visa in the first place is rather troubling. If you apply for a visa with a premise of "I'm going to be a tourist" rather that "I'm going to live with my boyfriend", you will have lied in a visa application -- a complete no-no, which will have deleterious consequences for your ability to return to the UK again if/when they find out. (And they certainly will find out if you apply for a residence card during your stay!)
If, on the other hand, you tell the truth in the visa application, you run into the problem that staying with a boyfriend for an extended/indeterminate period of time is not a recognized visitor activity, and can't be the basis for issuing a visitor visa.  Even non-visa nationals are regularly refused entry at the border if they let slip at their landing interview that they have such plans.
It appears that you best course of action will be to marry. Or else apply for the family permit and hope your relationship can qualify as "durable".

A:

Once you are in the UK, you don't need an EEA family permit anymore so the question is moot. If your relationship is sufficient to qualify for EU freedom of movement rights (and therefore for the EEA family permit), you can also appy for a residence card directly (it is not required for married partners, but it would be in your case).
There are however two difficulties:

Not being married, you will need to show your relationship is a durable one. If I understand you correctly, you have only lived together for six months, which might not be enough, even with strong evidence (the exact rules are complicated but usually, you need to have lived together for at least two years).
When applying for a Standard Visitor visa, your relationship might be held against you, because it casts doubt on the premise for your trip (tourism/temporary visit) and on your intent to leave the UK in time even if that relationship is not strong enough to qualify for the EEA family permit. 

Generally speaking, if you do qualify, the EEA family permit should in principle be easier/cheaper to get and certainly covers visits as well, so there is no reason to consider a Standard Visitor visa instead. It's obviously a big step but a marriage or civil partnership would also solve the problem.

