When the switch to 2e took place, it first required a special call, and the header was different. It allowed you to select old or new system. It was not a problem, although everyone did have to switch eventually.

 
Paul Thompson/Joy Hembel

Professor and Senior Scientist
Director, Methodology and Data Analysis Center
Sanford Research/USD
2301 E 60th St N
Sioux Falls, SD 57104


O: 605-312-6462   M: 618-974-0473  H: 605-332-1587  F: 605-328-0401


909 N. Charleston Circle
Sioux Falls, SD 57110
605-332-1587

From: Karl Berry <[log in to unmask]>
To: [log in to unmask]
Sent: Thursday, July 18, 2013 4:40 PM
Subject: Re: Time for \UseExplPackage? (Was re: l3keys feature request)

    ordinary users may have to change to a system

Since you ask for comments ...

My expectation (as I've expressed more than once to Frank, Will, and
others, by email and in person) is that any such latex3 format will be
invoked as "latex3", and "latex" and "pdflatex" will remain entirely
unchanged, invoking latex2e.  Thus no users would be affected unless
they choose to be.

Otherwise, as far as I can see, essentially the entire LaTeX-using world
will be broken, and that will harm everyone. 

Best,
karl