Slight Rewrite (developed it over a coffee break):

--Given

Declare @OldNode Varchar(5)
Select @OldNode = '90909'

Declare @OldPath  varchar(1000)
Select @OldPath = '//90012/90909/90000'

--and
--if(Len(@OldNode) = 5) -- This will always return true

/*

Using TSQL:

1)Assign the value of '90606' to a new variable called NewNode.
2)Using OldNode and OldPath replace the instance of OldNode in OldPath with the value of NewNode and assign the result to a new variable called NewPath. Print the results to Messages. 

*/