Changing from one chord to another comes with practise, the slower the better to get there faster. Take some regular exercise time (5-10 minutes every day without fail...) to finger the chord, then change to the next one, looking only for accuracy, not speed. Slow but sure is the way forward. This applies, whatever chords you're going from/to.
Intervals..? Easiest to imagine from a piano keyboard point of view, but it's exactly the same principle for all instruments. Take as a starting point the note 'C' on the piano. An interval describes the distance to the nexrt note played. From 'C', for example, playing the next 'C' on the keyboard gives an interval of an octave (there are eight notes in Western music, so 'octa' for eight...). From the first 'C', if we play the 'B' below the octave 'C', that an interval of seven (written in Romain as 'VII'...). Next interval from the 'C' to the next one down ('A'...), we have an interval of 'VI' (Yes, six...). Keep coming down from our first 'C', we play 'G' (interval 'V', of five...), then 'F' ('IV'...), 'E' ('III', or three...), 'D' ('II'...) and back to 'C'. Any note from 'C' can therefore be designated an interval ('VII' gives us 'B', for example...).
This is easy enough to visualise on a keyboard, as it's only the white notes, and from 'C'. The same 'logic', can be applied, however, if we had chosen to start with 'D', except, in order to play the same intervals as with 'C', we have to use some black keys. 'D' octave is easy enough, but a semitone down from 'D' (the same interval is 'B' starting from 'C'...) will be 'Db'. I'll leave you to work out what the other interval are, starting from 'D'; it's a good exercise to hammer home the understanding. Do it again for other starting notes, too, for fun.
It works in the same way for guitar, of course; on the open low 'E' string, the 'VII' is the Eb at the eleventh fret. Play an 'E Major' scale on that string and you'll see how it works.
Does this help..?