It was already described why it is generally considered unacceptable to alter a message to be forwarded.
There are however two practical exceptions I would like to share, to which me and the people I work with are widely used to. They both don't imply rewording or even reformatting the original mail.
1. Use the original mail as discussion wallboard
Unfortunately Stack Overflow/Markdown does not support coloring, so please consider bold and italic.
In this case, you will actually edit the text in the original mail, but will use colors or other visual highlights to separate your text from the author's original intent.
Original
From: orders@petfoods.biz
To: djechelon@nyan.cat
Dear /usr/local/ΕΨΗΕΛΩΝ,
about your order of 12kg of cat food, the shipping fee if $27.50. Please contact Mr. Dogg to plan a delivery to your house.
Your order ID is 28AC66F
Forwarded
From: djechelon@nyan.cat
To: manager@nyan.cat
Boss, my considerations below
Original
From: orders@petfoods.biz
To: djechelon@nyan.cat
Dear /usr/local/ΕΨΗΕΛΩΝ,
about your order of 12kg of cat food -is he really so hungry?-, the shipping fee if $27.50. Please contact Mr. Dogg to plan a delivery to your house. Could you text me his number? I lost his contact
Your order ID is 28AC66F
2. Highlight for visibility
In this case, you are not rewording the email, but only using bold or Outlook's highlighter (especially the default yellow one) to highlight something that the original sender didn't highlight, without rewording.
From: djechelon@nyan.cat
To: manager@nyan.cat
Boss, I made the order. Check it online
Original
From: orders@petfoods.biz
To: djechelon@nyan.cat
Dear /usr/local/ΕΨΗΕΛΩΝ,
about your order of 12kg of cat food, the shipping fee if $27.50. Please contact Mr. Dogg to plan a delivery to your house.
Your order ID is 28AC66F
In this case I highlighted the order ID to allow my boss (who is always hurrying) to easily spot the essential information.
And if the orignal mali si so badly speeeled or fOrmatted
You must either forward the email with its wording and formatting as it is, or if that is so bad it could damage the original sender, you may choose to quote pieces or summarize it to the audience without forwarding at all. Maybe one day someone from high will require to see the original mail; that's when you have to forward it.