How to change SSDT SQLCMD variable in pre-script?

This isn't possible. SQLCMD is just a pre-processor before the script is even sent to the server.

The answer you accepted is somewhat confusing sleight of hand.

It doesn't actually assign to the SQLCMD variable on the fly in any meaningful way.

:setvar myVar @sqlVar

Just assigns the string "@sqlVar" to the SQL Cmd variable (unnecessarily and misleadingly twice). This string then gets used in the string replacement to $(myVar).

All of this happens before the script is even sent to the server (and so obviously before execution has begun and the SQL variable is assigned any value)

The result of the script after replacing all $(myVar) with @sqlVar is as below.

This is what is what is sent to the server.

DECLARE @sqlVar CHAR(1)

SELECT @sqlVar = '1'
SELECT @sqlVar as value

SELECT @sqlVar = '2'
SELECT @sqlVar as value

There is no on the fly assignment to the SQLCMD variable at all. The only assignment is to the SQL variable.

Answer to https://stackoverflow.com/a/30086248/623190:

Cool! Be just aware of a pitfall:

DECLARE @sqlVar CHAR(1)

SELECT @sqlVar = '1'
:setvar myVar @sqlVar
SELECT $(myVar) as value

GO
-- SELECT @sqlVar = '2'
-- :setvar myVar @sqlVar
SELECT $(myVar) as value

result:

Msg 137, Level 15, State 2, Line 6
Must declare the scalar variable "@sqlVar".

If we cannot bring a value of SQLCMD Variable over GO (when it was assigned by regular variable) then there is no sense in using of the SQLCMD Variable within a batch.

Compare with the code that works:

:setvar myVar '1'
SELECT $(myVar) as value
GO
SELECT $(myVar) as value

And this magically works:

:OUT C:\Temp\SetVarTest.sql
declare @command varchar(100) = ':SETVAR myVar ' + cast((select count(*) from sys.objects) as varchar(10)); 
PRINT @command
GO
:OUT stdout
:r C:\Temp\SetVarTest.sql
GO
PRINT $(myVar) 

See more in Hacking SQLCMD Mode article.