With V3R2 of Enterprise PL/I, LIMITS(EXTNAME(7)) became the default (previously LIMITS(EXTNAME(100)) was the default). This default will ease your migration because this will make the default under the new compiler match what the old compilers always did - they had a limit of 7 characters in an external name and no option that allowed for a higher limit.
Also note that any n > 8 in LIMITS(EXTNAME(n)) requires the prelinker to be used or your modules to be stored in PDSEs.
Additionally, under LIMITS(EXTNAME(7)) (and under all the old compilers), if an 8-character name is declared as EXTERNAL, the compiler will take the first 4 and last 3 characters to make a 7-character name which it will pass to the linker. However, under LIMITS(EXTNAME(8)), the full 8-character name would be passed to the linker, thereby creating an incompatibility with the code generated by the old compilers.
For example, if the name DEZEMBER is declared as EXTERNAL, then under LIMITS(EXTNAME(7)), the linker will see the name DEZEBER, while under LIMITS(EXTNAME(8)), it would see DEZEMBER.
Consequently, for compatibility, do not use LIMITS(EXTNAME(8)) - use the default of LIMITS(EXTNAME(7)).
Finally note that LIMITS(EXTNAME(7)) applies only to PL/I names; assembler and COBOL routines can have 8 characters (exactly as they could with the old compilers).