Bro take the car to Honda Fort and change the transmission filter.
If the car is working fine at D3 it would be sole waste of time to change or swap anything. Check the ECU thoroughly .
Open up the transmission when changing filter and see the condition of torque converter and all 4 clutches. If only the torque converter is gone and eating up transmission fluid u can get one from kabar at Bilal Gunj for 8-10k. But its always risky to rebuild auto transmission with junk parts.
Check the fuel trims and clean the fuel lines and fuel tank. Check the fuel pump pressure.
A bit of tuning & u may get 8km/l and that is it. Even new CD3 Accord wont give more than 24MPG in city.
There is no harm in driving with D3 once u change the filter and get the fuel economy right. The car is big and its power to weight ratio is not very good. The non VTEC 105PS paired with 4 speed auto has to move 1,250kg vehicle.
Swap would easily cost u 120k even if its Bolt on. It will also reduce the Accord's value by 100k and u also have to spend to change the registration book or print new engine no. Not guarantee that it would be trouble free car forever or even that things could go worse.
In these cars swap is the last option unless the car refuses to move. If its KHi no car and u cant get the things right better to sell it at any showroom in KHi. It will fetch good money.
Sum things up if transmission filter change does not work change the torque converter or rebuild the transmission. Buy new OEM Honda parts from open market and get the job done from any Honda 2S or other mechanic who has done that before successfully. Honda Fort has done that job on a CD3 Accord before but ask for full estimate. They should not bill u more than 40k.
Overhauling the F18A engine is also not that difficult. Rings / Pistons & Valves r 15k approx new for China ones works fine for 5 yrs easily. Another 15k for labour and lathe machine time required 3 days.