Hi,
In my opinion when the retired partner is paid his capital balance plus profits accrued plus interest on capital or loan as the case may be, it will not be taxable again (As already tax is paid thereon either by firm or by partner).
But, if the retiring partner is also paid with the amount of Goodwill (which will be of course over and above the amount of capital & accrued profits), it may be taxed as Capital Gain