Not sure about engi's specifically but I've swapped the main class I play permanently, too. For the longest time I played(and FUCKING LOVED!!!) the thief. It was sooooo much fun to be control during dungeon runs and to dish out some nasty bursting dps. Then I heard about the revenant and it was pretty much everything I wanted in a class --> Heavy armor, magic user, skills with little to no CD's(like thief weapon skills), a huge pool of skills that are constantly used(like ele --> another class I played heavily), and mysterious lore surrounding its origins. My current problem is the build I use is not hugely popular or good in the raiding scene(the mode I focus on) unless they buff it a little more.

Do you regret swapping mains permanently? Do you regret the time spent on your original main?

Not really no. I absolutely loved my thief(still do) but another class has caught my heart. Now I have this other character that's relatively well geared(didn't start digging into ascended gear until HoT <-- when HoT announced I was about to craft some ascended gear then I went into hoard-mode once HoT was announced) and basically ready to hop into raids if my revenant isn't cutting it. My thief is also the class I still take into dungeons and guild missions(he's the one I have 100% map completion with).

All-in-all I'm glad to have spent so much time on something I enjoyed playing. If you truly loved your time with the engi then it was time well spent in my opinion. If you do decide to switch away from the engi, at least you have more knowledge about the class than most players ever will. That, in an of itself, is really valuable.