Welcome Paco. Good to have you drop by. Also welcome to SFDonovan. I must have missed your first post.
As for chord changes, there are two usual problems that folks have. First, they have a hard time making the chords ring out cleanly no matter how slowly they put their fingers down. SFDonovan's instructor gave good advice for that issue. Having your thumb on the back of the neck and having a gap between the webbing between your index finger and thumb and the back of the neck allows you to push your wrist forward on your fretting hand and get your fingers around to the front of the neck. You want to come straight down on the strings with your finger tips. You'll have to bend your fingers at the first knuckle up near the tip in order to do that. It's a bit unnatural feeling at first and takes some getting used to. Don't worry - it WILL come to you if you practice it.
The second problem people have with chord changes is getting from one chord to the next. There are a lot of exercises for this, but the one that helped me the most comes from Justinguitar.com, and it is what he calls "one minute chord changes." With these, you pick two chords that you have to change between to play a song. A couple common chords to change between when you're learning are G and C. So the One Minute Chord Change (again - check www.justinguitar.com for the source of this exercise) would have you start with your fingers off the fretboard. Start a one minute timer. Put your fingers down on a G chord and play it to make sure you got it right. Pick your fingers up. Put them down on a C chord and play it to make sure you got it right. Take as long as you need to put your fingers down to get a good, clean sounding chord. Pick them up and put them down on G. Play it. Then C. Play it. Every time take as much time as you need to get a good, clean sounding chord. Do this over and over again for a minute. Stop. How many times did you switch? At first, it might only be three times. But, if you keep at it, doing the changes back and forth for a minute at a time, and do that several times per day, you'll get it nailed down pretty quickly. You'll be doing twenty changes a minute before you know it.
Do the One Minute Chord Change program for each chord change you have in whatever song you decide to play. For instance, if you are playing a song that progresses from Em to Am to G to Em to D. Then you would do the One Minute Chord Change for Em to Am, for Am to G, for G to Em, and Em to D.
As for Justinguitar.com, I do not have any affiliation with them. They are a free resource (they'll take donations!) that many of us have found to be useful.
Good luck, stay the course, and most of all have fun.
Granted B chord amnesty by King of the Mutants (Long live the king).
If it comes from the heart and you add a few beers... it'll be awesome! - Mekidsmom
When in doubt ... hats. - B.G. Dude