As everyone else has said, it is best to get the most out of the system you trained in because then you have the "big picture" that you won't get trying all diff brands without training and support.
THAT being said, once you are experienced, things do change. People do convert from one system to another either because their needs/skills change and they want more/different things from the product.
Sometimes you just need a one on one with the current system. Sometimes you need to change the monomer within your system due to weather conditions. And sometimes, you outgrow a system and need to convert to another.
The thing is to utilise what you have to the best and if the best isn't good enough for you anymore, then its time to contact a quality manufacturer, try their product and if it suits, properly convert to that system.
NSI is good product. We trained in IBD in the dark ages, and I had done a conversion for NSI because the IBD wasn't doing it for me anymore way back when. Now, I was intrigued with Tammy Taylor and did a conversion there as well. Then I got out of the biz for 10+ years.
So this past year I took a foundation/conversion with EZ Flow because I wanted to get back into the biz and wanted to have a quality product available to me here in NZ. Doesn't mean I wouldn't/won't use NSI or TT on occasion, but my focus will be on EZF for the time being. (OK, Avolve is also got me hooked currently as well....)
The way I figure it, if you keep updated (something I didn't do for over 10 years, hence why I went foundation with EZF for my "conversion")...you will grow with the product you trained in and you may well "outgrow" it if you have utilised it fully.
the key is utilising it fully....hence the need for a one to one or brush up day.
Just cos you take a foundation course, or a conversion course doesn't mean that you are done with learning.....there is always something to learn, those "ah-hah" moments we all have, be it with nails or life. You have to be open to it and it opens up to you.