Switching

Absolutely. 
Thanks a lot for your help HTB

I understood your confusion and will try to ask a Juniper SE to check his feedback.
However, I built the BOQ for you case to compare QFX5120-48Y-AFO2 vs QFX5120-48Y-AFO.

The total for QFX5120-48Y-AFO2 is:

in fact you are right HTB when you telling that it must too quite to follow the vendor's advise with the Flex licensing mode (i.e QFX5120-48Y-AFO2). unfortunately, I meet a problem with that, because the Flex mode license (Premium or Advanced) don't permitted just 1 year subscription, it starts at 3 years. So, I want to subscribe just 1 year. 
in the same time the QFX5120-48Y-AFO is still available for quote and allowed choose of subscription support duration, in addition the JSB include the concerned feature, I mean the MC-LAG. Please see this Juniper public link: 
Licenses for QFX Series - TechLibrary - Juniper Networks

The link you shared says that I am not eligible to see it.

First of all, my advice is to quote the new model with 2, this is the way things will be moving and you have to follow the vendor's advice to be on the safe side and provide your customer with the best-recommended solution. 

For me without seeing the  F1ght3r reply, you have to get PFL or Advanced-1 as per the documentation. Please copy-paste the reply from F1ght3r here.

I think he replied privately.

Let me clarify a little bit this matter to avoid misunderstanding anymore. 

I am trying to quot via the configurator a "QFX5120-48Y-AFO" config, not the "QFX5120-48Y-AFO2". After discussion (see the link below) Daniel (F1ght3r) sent me some information/recommendation to quote that. 
Re: QFX5120 licensing and MC-LAG feature
So, I understood that no need to add license because the MC-LAG was included with the JBS. But, it's clear according to my shared link. Could you double check please? 

I don't see F1ght3r  reply, anyways you need an extra license to get MC-LAG.
Nowadays there is two types of the same hardware for example: QFX5120-48Y-AFO2 and QFX5120-48Y-AFO, The difference is the 2 at the end which is made to adapt flex licensing. 

I think you have the old series which comes without the 2 at the end and you need to buy the PFL license to get MC-LAG. Even in the link you shared, it is very clear that MC-LAG is mentioned as a licensed feature. 

Don't' forget to recommend the right answers :)

Hi HTB, 

thanks for your feedback, so, there is some misunderstanding between your post and the last post of F1ght3r who has explained the no need of additional license to get MC-LAG with a QFX5120-48Y. In fact, it seems depending on the model according these links : 
www.juniper.net/documentation/en_US/release-independent/...

Flex license (current license model):
www.juniper.net/documentation/en_US/release-independent/...

So, why this gap in the flow of information ? why these conflicting sources ? 
Really it's to hard to understand the licensing policy of one family product without talking about the real complexity of licensing policy in general.
So right now I'am very confuse.