What is it about HDR that requires the format extension?
There is additional metadata is that required to deliver the enhancements offered by HDR. HDMI 2.0a adds the ability for devices to be able to transmit/receive this metadata.
Will HDMI 2.0a require special hardware support or is this a firmware/software only upgrade?
This is dependent on manufacturers and their system designs. Please contact the manufacturers for detail of their system designs and upgradability.
Will HDMI 2.0a also be necessary for wide color gamut technology, and if so will receiver chips be necessary inside the TVs for this?
HDMI Specification Version 1.4 already supports a wide color gamut (x.v.Color) and Version 2.0 added support for BT.2020. Typically the content and the entire system, not just the receiver chip, will need to support the wider color gamut to fully deliver the benefits of this feature.