David
You are asking about altered tunings. First of all, the guitar is not tuned to a G as you mentioned. The banjo is tuned to an open G but the guitar's standard tuning E-A-D-G-B-E is not an open G or an open chord in itself.
The tuning of D-A-D-G-A-D is a popular altered tuning for guitar. Some have incorrectly called this a modal tuning. In fact, the tuning is to:dry: a Dsus4 chord. This is not a popular bluegrass tuning. Other open tunings on the guitar may include an open G tuning or a Dropped D tuning.
The drawback to playing in open tunings is that you have to learn new (different) chord forms. This is not something beginners are prepared to do, until they have mastered the standard tuning chord changes.On the plus side, the altered tunings allow for variations in your sound but they also require you to re-tune the strings for each song in a altered tuning. Try it a couple of times and you will see how old changing your tuning becomes.
To capo (the open tuning you mentioned) up three frets would change your open chord to an open fsus4 chord. The bottom line: if you really don't understand what makes up an sus4 chord, then it is probably premature for you to explore an open tuning as well.
Finally, this is not a simple matter of modulating, since the tune you suggested is written in a sus4 tuning.
Hope this addresses your questions.
Dave
