As far as the tray and connectors, the kx155 shoulf be a "plug and play" but you will have to add the pins for the G/S, and depending,KX155 or KX155A may need to change the backplate.
If you have a 155 without G/S you'll need to add the wiring,from the radio to the indicator, for G/S and an antenna.
Make certian that the connector is compatable with your new indicator. There are two indicators compatable with the kx155, but they require different connector.
The KI203 does not have G/S, the 204 does. The KI203/204 has rectilinear movement, which (to me) gives a better indication of situation, than the pendulum movements of the KI 208/209 units.
The KI203/204 use the same connector, and the KI208/209 use the same connector, but different from the 203/204.
That said, It shouldn't be too big of a deal.
PM if you need a backplate for KX155A.