The correct way to migrate all content from 1.4 to 1.7

[TinoArts]

Hi,

so the scenario in short terms is: I have a very active e-shop running Presta 1.4.9, which has tons of custom functionality, customized modules and so on. Apart from that tons of products, users, media, orders et cetera.

Apart from that I am building a new store running 1.7.5. When it's ready, I plan to move everything from the old store to a new one. 

What would be the best way to do it, with as few errors and bugs as possible?

Thank you.