How can I do this? For example, I am using a system and I am going to switch to another system. How can I perform this transfer?
You can directly migrate your forum system to another forum system. The last I checked, you could migrate between most forums, but I don’t know what the current situation is. If you tell me which forum system you are currently using and which forum system you want to migrate to, maybe I can help. 
from esotalk to phpbb
Sir, I looked around the internet a bit, but I couldn’t find a resource that explains it in detail. However, I did find individuals/companies that do this for a fee. Their pricing varies depending on your number of topics and users. You might want to check out the relevant pages of the ProfProjects and GConverters websites.
If you only need to get the topics, you can have a bot built to scrape them.
I’ll check them out shortly, sir.
It varies depending on the forum systems. For example, if you are going to migrate from phpBB to SMF, you can find various methods by doing searches like phpbb to smf.
It seems like a somewhat difficult process.
It’s not a well-known system, sir. I guess I could do it if it were.