You want to modify
\ExistingFolderName
into
\%year%\%album%\%artist%
like
\2026\Waterloo\ABBA
?
Maybe this FAQ topic "How to create directory structures based on the tags and move the files?" can help you using the 
Converter > Tag - Filename with a format string:
How to create directory structures based on the tags and move the files?
When you're using a backslash in a format string, Mp3tag will create a directory from the part of the format string in front of a backslash.
Here is an example that creates new directories under
D:\Music:
D:\Music\$left(%artist%,1)\%artist%-%year%-%album%\$num(%track%,2). %title%
The same is also possible using relative path names (instead of an absolute path in the example above) which creates the new directory below the current working directory:
%artist%-%year%-%album%\$num(%track%,2). %title%Please note, that the whole range of Mp3tag's Scripting Functions can be used in the format string.