VBA: Find and replace based on value of cell

I am trying to find and replace data within a cell based on the value of a cell in a different column.

This is a 3 part update:

Step 1 - Look through column B for one of the agent IDs listed in the macro  (It would need to look for multiple values)
Step 2 - If there is a match to the listed Agent ID, replace the value in column L with X (this would also be stored in the macro as one value)
Step 3 - If there was a change made to column L, change the value in column A to "Y"

Hopefully this makes sense. It would then be stored to my personal macros. So when I export the user file for our phone system, I can make the changes and then upload it back into the phone system for quick updates to the agent skills.

I attached a sample file of what I export.

Here is the list of agent IDs (Column B) it will need to look for:

15344995
15345008
15344996
15393269

Here is the value that would need to replace the current data in column L:

4349850|3;4349892|2;4349893|1

I can do a basic find and replace macro but do not know how to make it based on the value of another cell.

Any help you can provide will be much appreciated!