Lookup Table change with time

This is definitely a plausible answer; however, this would create a very large lookup table.  Especially as time goes on. I also need this accurate by the month. If there really is no other way I can definitely do it like that. But still looking for a solution that requires a bit less expanding.

I think I worded this kind of poorly initially. So allow me to restate my problem.

Vvelarde 

I have a lookup table set up like this

Employee ID    Employee Name   Budget Group      Job title

1                       John                     Training                Trainer

2                       Mary                     Management       Manager

3                       Greenberg            Management       Assistant

Employee ID's and Employee Names do not change. How can I make this Dynamic so that I can change someones info without affecting data where they are under a previous job title & Budget group?

The source data is formatted like this using a folder that updates monthly

Date        Cost Element      Employee ID     Cost

x              x                          x                       x

x              x                          x                       x

x              x                          x                       x

So for Example, John is going to move into management into Manager. But this goes into effect in March of 2019. I can't simply add a new selection like shown below because that will make it a many-to-many relationship. I also cannot change his budget group and job title because that will misrepresent his spending for previous years as a trainer and place that money spent on the manager budget when it shouldn't be. How may I approach this?

Employee ID    Employee Name   Budget Group      Job title

1                       John                     Training                Trainer

1                       John                     Management        Manager

2                       Mary                     Management       Manager

3                       Greenberg            Management       Assistant

Thank you,

Michael