I have a requirement where a text field char size(255) is not sufficient and I have used that text field in various codes.
If I change datatype from Text to Text Area (Long) will my other functionality get hampered ?
If your text field is referenced anywhere in Apex code, you won't be able to change the data type until all references are removed.
See this answer for more information on exactly what is involved in doing this.
Other than that (admittedly considerable) issue, there should not be any problems changing from a text field to a long text area. You should not experience any data loss as they are compatible fields.