Re: Old Optics in New Switches, Any Benefit to Replace Them?

Hi,

First offtopic -

depending on intended use EX3400 may or may not be most suitable replacement for EX4200. EX4300 is direct replacement for EX4200.

If needed check here:

https://apps.juniper.net/feature-explorer/compare-multiple.html#pkey=30504200%7C30504300%7C30503400&platforms=EX4200%7CEX4300%7CEX3400&stat=0.01181865831171991

Regarding SFP+ - 

if cost is no issue, buy new SFPs as you get warranty with them. But nothing is also wrong with continuing use of existing ones. I don't know about possible advancements in new HW revisions though ...