I have changed systems very few times, fortunately without much regret, but it can be a costly exercise. Despite declining camera sales there is an incredible range of equipment available today, SLRs are at the peak of their evolution and mirrorless systems are now a mature and usable system for many disciplines of photography. There is so much choice and a significant number of professional and hobbyist photographers are making changes.
I started photography when I was still at school and as I write this, some thirty years on and a bit resistant to change, have spent more time with film than digital. Aside from upgrades through generations of camera bodies, my major system changes have been from Nikon film SLRs to Leica rangefinders, then from film to digital, initially with a Nikon D90 that I used side by side with my Leicas and then moving fully from film selling my Leica equipment and buying a Nikon D750 and full frame lenses. Most recently I am moving to micro four thirds with Olympus cameras and lenses.
The only camera I truly miss is my Leica MP. Despite the high cost of Leica equipment however, this was probably the only system that held its value throughout my time with it.
Assuming you have looked carefully at what is driving you to change system, here are my suggestions.
Rental - this is probably the best option, allowing you to get your hands on the system you want to try out without limitations, look at the photos and try it in real life situations. In London, both Fixation and Calumet have a good range of cameras and lenses, in the USA, Borrow Lenses have an excellent reputation.
Buy second hand, previous generation - this has always been my preference, if you choose wisely you can get the feel of a system over an extended period and sell at little or no loss if it doesn’t meet your requirements. My favourite dealers for used cameras in London are Aperture Photographic and Camera World. Both price sensibly, offer good warranty terms and check items thoroughly before selling them. Even if you are considering the latest release, the previous generation will normally give a very good indication of handling, ergonomics and any quirks.
The best alternative to getting hands on with the system, is to see what images it is able to produce. Of course you won’t be able to form an opinion on ergonomics, but you’ll see what it is capable of. Flickr has groups for pretty much every lens and camera combination across many disciplines of photography. Better still, try to find some RAW photos from manufacturers’ sites or photography forums if possible from the “prospective” system and run these through your usual editing or post production workflow.
Engage with photographers you trust - whether this is friends and family or other photographers in the public domain, I have always been pleasantly surprised at how happy other photographers are to provide honest advice and guidance on the equipment they use. A good start tends to be the brand ambassadors who generally have active social media presence and enjoy interacting with others.
I also think it is worthwhile speaking to dealers. Maybe even put a call in with the manufacturer’s customer support department prior to purchasing to see if it is up to scratch.
Banish preconceptions - there is too much talk of “this vs that”. Different types of photography benefit from different equipment but there is no hard and fast rule. MFT is great for fast action, wildlife etc., Fuji is building a real following with street photographers but in fact I have gone the Olympus OM-D route. What has been developed with speed in mind for sport, brings huge benefits to the speed required for street capture.
Look at photos you have taken - are they possible with the limitations of what you are looking at? Are they possible with the most basic version of that system or are you stretching it beyond its sweet spot?
Look at the technical aspects of your favourite photos from others - do they have tiny DoF (you may need a larger sensor, full frame or medium format) do they require super long lenses? (Forget Rangefinder), do you need a well developed lighting or flash solution? What’s the final output - print or screen - don’t chase pixels if you don’t need them, but don’t forfeit them if resolution is required.
Don’t forget the lenses - I find myself attracted by the cameras, whether it is the functionality or ergonomics, they are the focus of reviews, discussion forums and general attention. However, in reality it’s all about the lenses, they last through successive generations of bodies and are fundamental to quality and the type of pictures you want to make. While the major DSLR manufacturers have extensive options, more modern systems are much more limited, particularly with specialist optics including longer lenses, macro lenses and rarer requirements such as tilt-shift lenses.
Above all, enjoy the change, if it gets you taking more photographs then it will be worthwhile.