First things first. If you want to upgrade, is better if you do to 18.x instead of 17.x. You will have longer lifecycle (17.x will be out of support in 1 year as per modern lifecycle).
Back to your question. There might be several ways to move data from your C/AL table into your AL table.
The cleanest and bulletproof one
1. Once you have an up and running version 14.x, it is time to do some magic and move the content of C/AL custom table fields into AL table extension fields
a. Refactor your AL extension for 17.x (18.x) in order to cope with 14.x and be sure that AL table extension fields does not have the same ID and Name of the C/AL custom table fields. Obviously captions and all the rest stays the same and AL code is referencing the AL field names and IDs.
b. Publish and install the AL extension
c. Create another AL extension (e.g. Called TableExtensionDataTransfer extension) that has a dependency on the extension at point a. e.g.
"dependencies": [
{
"id": "499ec319-ef73-463a-8d31-d66a58e1234a",
"name": "My Extension with Table Extension",
"publisher": "yourpublishername",
"version": "1.2.3.4"
}
],
This will download also symbols from the extension at point a.
Within this extension, create a codeunit that move the content from C/AL custom table fields into AL table extension fields and a page (or page extension – up to you -) with an action to run this codeunit.
d. Publish and install this TabelExtensionDataTransfer
e. Run the client (windows or web, it does not matter) and perform the data transfer
f. Check the result in SQL Server and be sure that data are correctly moved from C/AL custom table fields into AL table extension fields
g. Uninstall and unpublish in clean mode TableExtensionDataTransfer (silent, swift and deadly: leave no trace)
h. Take an FOB file with all standard table and synchronize with Force (this will delete all custom C/AL fields)
i. Regenerate symbols reference to have back the right symbols in Object Metadata table
3. Now that 14.x is completely standard application and you do not have any C/AL Customization but AL extension, you can perform the upgrade from 14.x to 17.x (18.x) as per Upgrading Unmodified C/AL Application
