For some people it's true. Because different hearing aid manufacturer's use different algorithms, different attack and release times for compression, different noise reduction strategies or frequency lowering, some people do have difficulty transitioning between manufacturers as their brains have adapted to how their old manufacturers process sound. That said it's not impossible and some people do it with no problem.
Regarding your current situation, depending on the age of the aids, they may no longer be eligible for loss & damage warranty replacement and if they are old enough the provider may no longer be able to obtain an identical model to match with your old one which may lead to the recommendation for a new pair, especially with newer technology, the aids often communicate with each other (and depending on bluetooth capabilities) and it is nice to have a matched set. I'm not 100% certain why you were feeling the need to change manufacturers to replace one aid?
Hope that's helpful.
It is important to have the same circuit for both sides. If your old aid cannot be reordered or replaced then you will need two new aids (even if the same manufacturer) The main reason is that each circuit sounds different, and wireless aids work together but only if they are the same.
You can transition to any manufacturer you want, every manufacture sounds different and it is a matter of preference. I both wear hearing aids and dispense them. I have my personal favorites, but manufacturers come up with new stuff all the time. I have noticed that the past two years the hearing aid industry has really up'd their game in quality.
Ask your clinic to borrow a demo set of aids, if you don't like it then try another... that is what I do with my patients who are unsure of what they want. By giving people options (I carry all major brands) they are more likely to be happy with the hearing aid purchased and even more likely to wear them! Win/Win!!
Replacing a lost hearing with an identical match is always the best choice. If the lost instrument is over 5 years old it may be worth considering and discussing new technology in both ears. Changes in hearing level and lifestyle will usually be your best identifyers if new technology is the proper course of action.
This doesn't really make sense at all. First, there is typically Loss & Damage insurance on most hearing aids for a certain period of time. Even if this is expired, manufacturers usually have older generation devices to replace the one that is lost.
While it does make sense to stick with a manufacturer's devices that you are happy with, by no means does this mean that you couldn't transition to another brand. People do this all the time and some end up liking the different brand better.
The brain is very "plastic" meaning that it will change based on the inputs we provide it. Even if you didn't like a new brand of hearing aid initially, your brain would change to accept and appreciate the new sound.
Every manufacturer has their own fitting formulas and ways of analyzing incoming sounds signals. Because you previously wore Resound hearing aids your audiologist probably recommended that you continue using Resound hearing aids since your brain was accustomed to their sound.
With that said you are not forced to keep using Resound! If another hearing aid manufacturer has features you are more interested in then now is the time to transition over.