There are many area of libs in AEM 6.4 which has been greyed out or depricated.
As per the the document suggested by Adobe Adobe Experience Manager Help | Sustainable Upgrades , there will be no further enhancement for these areas and greyed out area which has mixinType granite:internal and granite:final cant't be overlaid or not suggested to overlay.
In AEM 6.3 , i have done overlay and then customization from these area because there was nothing like greyed out or depricated functionality which has been introcued in AEM 6.4. So in the process of migration from AEM 6.3 to AEM 6.4 what should be the propsed solution so that overlayed content should not break even further.
If ill go for creating another structure in AEM 6.4 like AEM 6.3 libs and overlay content from that area or if ill think to move complete AEM 6.3 libs content to project or overlaid location then it is taking a lot of time and not sure if this will work out in future as well.
This is a migration scenario from AEM 6.3 to AEM 6.4.
Could someone provide suggestions on this or should i raise any day care ticket for such kind of issues?
You may open ticket(s) for specific use cases. In general, the greyed-out area should not be overlaid/extended. That means you'd have to rewrite those use cases in a different way (re-engineer) to achieve what you're doing today. This may be as simple as using an OOB property of granite/coral3 libraries or could involve entire redesign of a particular use case. You could also mimic the existing functionality and then create a wrapper of your customization on top of it. The approach may vary for each use case.
There is a pre-upgrade utility provided by Adobe to understand the scope of changes required - install that package and estimate the scope/effort of changes required.