find and replace, in a proper text editor, because you would visually confirm each change, is absolutely the way to go.
a blanket find and replace is not going to be 100% accurate due to coding styles and stuff;
for example: if you search for CREATE PROCEDURE to repalce it with ALTER PROCEDURE
any differences in whitespace betweent he two words, or where a develoepr wrote CREATE PROC instead;
WITH ENCRYPTION needs to right before the AS keyword, but it depends on coding style, whether you can find 'nAS, or whitespace around it
-- Create Encrypted SP
CREATE PROCEDURE uspEnc
--help us help you! If you post a question, make sure you include a CREATE TABLE... statement and INSERT INTO... statement into that table to give the volunteers here representative data. with your description of the problem, we can provide a tested, verifiable solution to your question! asking the question the right way gets you a tested answer the fastest way possible!