akaram

Did some calculations:

Internal resistance of Standard Sanyo Cadnica 1250 mah cells approx. 5 mOhms/cell. For a 6 Cell pack this is 30 mohms = 0.03 Ohms
Internal resistance of Good Lipo Cells approx. 15 mOhms/cell. For a 2 cell Lipo this will be 30 mohms = 0.03 Ohms
Total impedance of both batteries will be 0.03 + 0.03= 0.06 Ohms

Considering the worst situation when your ECU battery is less than half-charged at a voltage approx. 7.8v (1.3v/cell) and the LiPo pack fully charged at 8.4 volts there will be a voltage difference of 0.6 volts.

The current I=V/R=0.6/0.06=10 Amps

This is the worst case in an ideal world. If you ECU battery is at 8.1 volts (1.35 v/cell or approx 80% charge) your current will drop to 5 Amps which is already perfectly safe. Moreover normally your ECU batteries will have much higher internal resistance.

An easy test will be the below:
Discharge your ECU battery to below 50% charge (you will never actually fly ur jet at below 50% ECU batt charge), hook it to your charger and increase the charging current gradually till the battery voltage on the charger output reads 8.4 volts and note at which current this takes place. If this current is still within the safe charging range for the ECU battery then you can be sure that this current will never be exceeded when connecting a LiPo on the field.
Normally the maximum safe charging current for NiCd is 4C or 5 amps for your ECU battery.

If I were you I would just change the ECU battery to Lipo otherwise do the above test.