I would suggest you wait on both EX2200 and EX3300, especially since all is working well. What is driving people to upgrade legacy EX products, EX2200/3300/4200/45xx, is the fact that 12.3 release is going End of Software Support (ESS) June 30, 2017. This still gives you a 6 month window from today.
The next (and final) release train for legacy EX will be 15.1. I would recommend ONLY using either 12.3 or 15.1 base release for legacy EX products. The one exception is for EX45xx and MAC-SEC enabled deployments, where 14.1x53 must be used.
Within the next 6 months, there will be newer R relases of 15.1 coming out. Depending upon timing and your window opportunity to upgrade, you will need to eventually choose one of these. The 15.1 release for legacy EX products will be supported until End of Life (EOL) of legacy EX products. To date only the EX4500, for which the EX4550 is a replacement, has been announced End of Sale (EOS). EOS is the 1st step in any Juniper EOL product decision. Recently the EX4200 was made ROHS2 complaint, required for new sales into Europe, so EOS announcement does not appear to be happening soon. Just FYI, not fact!
As for EX4300, I would suggest an upgrade to 14.1X53-D40, as this release has a number of important fixes as well as enhancements, such as support for HDX communication for 10/100. For the newer EX platforms, EX4300/4600/3400/2300, the next release for all these products will be 17.1. The 14.1X53 train will eventually go ESS some time in the future; I do not know exactly when.
In 2017, Juniper is also going back to prior 4 releases per year model, so you will see 17.1, 17.2, 17.3, 17.4, then 18.1 etc. This will apply to PTX/MX/ACX/EX/QFX product lines, basically everything Junos besides Security/SRX. I am not sure when SRX will rejoin the mainline, but for now SRX will stay on X releases.
Hope this helps