Find And Replace Text On Worksheet

I need some vba coding which will find and replace text on a worksheet. This would normally be straight-forward, however some text needs to be replaced by text which already occurs in the worksheet, and without it changing as well (if Find/Replace can perform two Find/Replaces at the same time???).

Here is the detail and what I am trying to accomplish: I have two worksheets each with a table of data.Worksheet 1 has a lookup table with three columns of data (column a and b are lists, with c being a formulated column which is dependent on the user selecting either column a name or column b name). I have done this easily enough using data validation on cell $C$1.Worksheet 2 has a user input table which column 3 is a dropdown validation using the named range "UsedName" from Worksheet 1.

I want the selected dropdown names to automatically change when the user changes Old Name to New Name (and vis-versa) on Worksheet 1.The list of values in the data validation dropdown list change well enough, but not any of the existing returned values. When I tried to use vba coding to Find & Replace, I run into issues because (Substanital and Important) are used in both instances, but at different levels with different matched names Important/Relevant.

Worksheet 1
A B C D
1 Cell with dropdown list New Names
2 (Old Names, New Names)
3
4 Old Names New Names Names Level
5 Equal Equivalent Equivalent 1
6 Dominant Critical Critical 2
7 Important Substantial Substantial 3
8 Substantial Major Major 4
9 Relevant Important Important 5
10 Minor Irrelevant Irrelevant 6
formula for column C =IF($C$1="Used Names", A5, B5)
column C is name ranged "Names

Worksheet 2
(Worksheet 2 has another table which has various data. One column which is a drop down list being pulled from Worksheet 1)

A B C D
1 Bob 25% Critical 19
2 Frank 60% Important 33
3 Ellen 40% Substantial 5
4 Ellen G 20% Substantial 12
5 Gary 55% Equivalent 100
6 Jo 50% Major 8
7 Peter 40% Important 22
8 Kim 12% Equivalent 30
9 Shelley 75% Substantial 15
10 John 75% Critical 90

so I want column C to automatically change from these New Names to the corresponding Old Name of the same level. So Critical would become Dominant, Important would become Relevant and Substantial would become Important.