How to move from self-written CMS to Opencart?

Question

CMS

There is an online store - it was originally made on CMS ShopScript, but later the engine was completely rewritten by the programmer.
A lot of problems follow from all this - a lot of things the programmer no longer remembers how he did it, respectively, he cannot remake or finish it - he is afraid to break it.
The admin panel is very convenient for the site owner, but because of all this, the client side suffered.
The site has good traffic and positions in Google, which I would not want to lose, as the site is being redesigned in order to increase traffic, customers, purchases, positions in PS.
But let's say there are a lot of page duplicates, the old design, the lack of a mobile version, and a million more reasons gave reason to think about moving to Opencart.
So the question is - where to start moving? There is a programmer who did all this and is ready to participate, there is a new programmer who is ready to deal with opencart and integration.
The question is exactly where to start, what goals to set? Important - maintaining the client
base (balances are maintained there, for example, the history of purchases), maintaining positions in Google (I would like to change the URLs, but will the 301 redirect save? Or will the URLs have to be left the same during the transfer?)
uploading orders to the old admin panel, but how then to link products, prices and customers?
Who faced similar tasks - tell us about your experience, who started with what, what goals and objectives were set, what mistakes were made in the process and all that. Thanks in advance.

Answer 1

It is impossible to give an unambiguous answer without seeing the structure of the existing project.
I didn’t do SEO optimization (I won’t say anything here), but here you need to take into account the fact that urls can change depending on cms and how they are built now.
Transferred the site from pure samopis to magento.
The most important (and probably the most difficult) was to transfer goods, orders, users from the database.
I made a script that pulls all the products on the self-written, and the second one for adding products programmatically to magento (because if it's just import / export, then you can not take into account all the dependencies in the tables) it's a long time, but all the entities were transferred normally.
The magento template was bought +- of a similar structure and was already finished locally.
The transfer was without a redirect, the new site was on a different server, they were simply replaced by NS on the domain

Answer 2

This question is old, but still important.
The transfer must begin with the deployment of the site on a separate domain for development, as a rule, this is a subdomain of the main site - licensed modules of different cms support this for free, or configured on another server for the same domain.
Next, you need to transfer products, categories and content with meta tags, all links on the site, and also prepare a list of 301 redirects for htaccess, since there will most likely be other urls, or get confused and migrate the url generation algorithm. During the migration, the content must be exactly the same as on the old site. All points in this paragraph are critical and mandatory.
The transfer of orders is an important matter, and some customers may swear a lot, but the majority treats with understanding, the main thing is to fulfill obligations and be in touch. When transferring orders, it is also necessary to transfer personal accounts with a password reset and a notification that our site has become better, more convenient and safer, so you need to update passwords.
Further, there will already be nuances related to specific cases, for example, infosmsk (sms notification service) does not have a module for opencart 3. Or Yandex.Checkout - you can have the old method connected, and the new cms with the module is only for the new method, which means one or a couple of days, payments may be difficult to accept while you are being switched. If you have your own mail server, you need to reconfigure it accordingly.