OK, I have a ticket from Atlanta to Paris, Paris to Prague...and the reverse for the return trip. There is a very steep change fee, but now we'd like to travel from Paris to Berlin, and on the return travel from Prague to Paris. So, we were thinking of skipping the last leg of the "To" journey and book a seperate Paris to Berlin ticket for far less than the change fee of the original ticket.

Does anyone know what would happen if we skipped the Paris to Prague leg? Would that mess up or cause to be dishonored my return ticket from Prague to Atlanta? Of course, this is all assuming I have no checked bags. I hope this question makes sense!

They will almost certainly cancel all remaining segments if you no-show on one. Pretty much universal policy among airlines, and spelled out in the T&Cs as a rule.

Meant to add, though, that you can call them and get an accurate read on the change fee or their willingness to waive it if you're not actually making a destination change, just shortening the outbound trip. I wouldn't expect them to approve the change without the fee, but asking is free.

call them and just ask to keep your return trip confirmed even though you will not show up for Paris to Prague so you do not have to change anything

You can ask, but they can say no. Unless you have a good reason/excuse.

Do not play this game, as you may lose a lot.

My understanding is that if you miss a segment, all remaining segments are cancelled automatically, without human intervention. So if you have called and asked a clerk to keep your reservation, they will refuse if they know what they are doing, and try to sell you a revised ticket, with penalty and very likely with a higher price. Even if you convince some clerk to annotate your reservation, I think that annotation will be trumped by the computer's automatic cancellation, which will leave you far from home, buying a last-minute one way ticket to get home. Have plenty of money with you. Paying the change fee now is probably your most economical move. I have heard of instances where the clerk waives the change fee, but they are really rare.

Just adding to all the other replies,

your return will be cancelled.

Not a good idea.

I was afraid of this, but wanted to verify. Thanks for all the replies!

Are you flying the same airline for all segments and the new requested Paris to Berlin segment ? You could still ask them just to cancell the reservation for the Paris to Prague segment so it will not be a no show and nothing of the return trip will be cancelled automatically by the system.
After all you are adding a new Paris to Berlin ticket, this means additional money for them.
I do not see any reason why they should say no !!

Simple - fare rules

The ticket may have priced out a great deal more if the middle segment was not there originally.

thanks

I get the point, nevertheless they still might say yes. I would try anway.
Rules are not flexible but human beings are !

They will reprice it for the OP and there is a good chance the agent may put a note in the OP's PNR if the OP declines the new fare. By asking one may simply wake up the sleeping guard. Not worth it, IMHO, unless you are either willing to live with the new fare or just complete the trip as booked.