So guys
I am not going to change anything in my database because after reading the docs I believe that there is nothing wrong to use int(200), double(200,2), varchar(200) and it will not going to make me any trouble as per my requirements.
Still I want to add one solution for my original question-
we can write a php script to change data type in the entire database together with the following code-
$loop2=$con->query("SELECT COLUMN_NAME, TABLE_NAME, COLUMN_DEFAULT FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_SCHEMA = 'database_name' AND COLUMN_TYPE = 'int(200)'");
while ($row2 =mysqli_fetch_array($loop2))
{
$row1=$con->query("alter table database_name.$row2[TABLE_NAME] change column $row2[COLUMN_NAME] $row2[COLUMN_NAME] int(20) default '$row2[COLUMN_DEFAULT]'");
}