The basic idea behind 30/30s or 40/20s or 30/15s is something like this… Lets make it really simple, say a 250W ftp and doing 3x3-min at 120% is your limit. That is doing 3 minutes at 300W, and after 3 sets its 9 minutes at 300W.
Now you do 30/30s, and lets make it 3 sets of 6x30-sec on / 30-sec off. However instead of doing 300W, you can repeat 350W (140% FTP) for each of the 30-sec. Each set is 3 minutes at 350W, and 9 minutes total at 350W.
Different intervals, however in this simple example you accumulated 9 minutes at 300W with one vo2max format (3x3-min), and 350W with the other vo2max format (3 sets of 6x30-sec/30-sec). More power with the 30/30s.
Some discussion on adaptations here: Micro-Burst Intervals: The Most Effective HIIT for Increasing Power and VO2Max
In any case, vo2max work is suppose to be done with max repeatable power, instead of constraining it with erg and % ftp. So it has nothing to do with an ftp update, unless you determine max repeatable power in advance, and then reverse that into a %ftp if you insist on using erg.