Conversion of VGO (70 Deg C) storage cone tank to 140 Deg C VGO tank.

I don't have a complete list of precautions. However there are two things I would check.
1. Make sure associated piping can safely handle the expansion due to the higher temperature.
2. Make sure the tank is dry before going above the boiling point of water. When you get the tank in hot service, keep the contents warm enought so that water does not settle out. Otherwise there will be a serious risk of foamover. HAZOP at

To keep the VGO at 150 Celsius you'll need improving the thermal insulation.

Probably it would be worthwhile to check the TAN values -from naphthenic acidity levels sometimes present in VGOs and acidity that can develop by oxidation with air- and whether these could be corrosive to carbon steel at the new operating conditions. Sludge can form from condensation of oxidation products. If the availability of oxygen is limited by diffusion, there is always the possibility of thermal cleavage of a hydrocarbon chain with the release of hydrogen. Thus, if materials and construction are acceptable at the new conditions, should also consider: inert gas blanketing with breather.