Changing the purpose of stay. Is it possible?

I'm algerian and I came to the netherlands last december for one year traineeship. I have a temporary residence visa (type1)
I met my boyfriend who is dutch 2 months later, and we have been in a happy and exclusive relationship ever since. We are not up to do a long distance relationship and it would be hard for my boyfriend to move with me to Algeria since there we won't be allowed to even live together.

This is why we are considering to apply for Partner/Spouse based VISA, but we got confused during the process.
We thought at first (due to our long and excessive research process) that we have to register a partnership at the city hall before applying for the visa. It turns out it was not necessary, but we have already declared an intent to register our partnership.
We are worried if this (the declaration)  would affect our VISA application which is changing my traineeship visa to Partner based visa.

We do fulfill all other requirements such as income from his side, I have my proof of being single legalized and stamped from the relevant authorities and so on, I have met his family, and we have already been on a trip together etc


Anyone who have had an experience from this kind?  Is it legit to change the purpose of residence this way?