Couple of clarifications below. #1, ELS was a way to get more CLI syntax consistency across all Juniper products, that run Junos. Namely EX/QFX, MX/PTX/ACX, and SRX. For SRX branch follows like EX, while HE/Mid SRX is like MX.
ELS/non-ELS for EX is model dependant, not SW dependant. You can not change SW and get a different format of the CLI syntax. ELS is support by all newer [Broadcom] based models, while non-ELS is used with older legacy [Marvel] based models.
Both EX9200 and all [current] QFX 5K/10K based models only support ELS, from day 1.
The ELS Translator tool has been decommisioned, so no longer available.
MX is the one product that moved to ELS in SW, but MX was basically ELS-like always. MX L3 is the same as ELS L3, and MX (and EX9200) still only support "bridge-domains" for L2, while EX/QFX ELS supports "vlans".
MX LAN Mode (https://www.juniper.net/documentation/en_US/junos/topics/task/configuration/configuring-mx-enhanced-lan-mode.html) which is now an obsolete concept (has not really applied for last 5+ years), has nothing to do with ELS Syntax on MX. MX-ELM has basically been replaced by newer Flex Licensing model.
I believe the above is correct. Just to repeat, for EX 4K series and QFX 5K/10K, you can not change from ELS to non-ELS or vice-a-versus via any SW change. Branch SRX are similar, in the older 2xx Series is non-ELS, while new 3xx series in ELS based.