From what I read, you should replace the TIM (Thermal paste/pad) only when you notice your Laptop running hotter than usual under normal circumstances after making sure your laptop was cleaned of all dusts from the fans and exit vents. If by cleaning your Laptop's vents and Fan areas and your laptop's CPU/GPU temperature stays within normal range than there is no need to replace the Thermal paste or pad.
Here is a very good review of over 80 Thermal Pastes by Tom's Hardware - Best Thermal Paste for CPUS and GPUs - 85 Products Tested, Compared . It gives a very good explanation on how Thermal pastes/pads work, how to correctly apply them and at the end of the review it shows the 85 brands concerning Viscosity and Usability of each brand.
Generally if your CPU/GPU has a thermal pad you should replace it with the exact thickness thermal pad unless the Thermal paste was made as a replacement TIM for thermal pads. If the manufacturer used Thermal Pad instead of paste is probably because there is a gap between the heat-sink and CPU/GPU surface in which Thermal Paste won't be able to cover the entire gap. It the thermal Pad is very thin, then you may be able to use Thermal paste but need to find one that is made to replace Thermal pad like K5-PRO thermal paste.
Read from one thread this paste is good if the gap is .5 to 1 mm thick:
k5 works ideally from 0-0.5-1mm
Copied from Google Search:Can you replace thermal pads with paste?You should replace thermal pads with new thermal pads or with a thermal paste that is specially designed to replace thermal pads. Do not reuse old pads and do not replace pads with normal thermal paste like Arctic Silver. Both will lead to overheating and result to GPU or other BGA failure.Feb 14, 2011
This from Overclockers website concerning Thermal pads: Why not to replace your laptop's thermal pad with thermal compound... - Overclock.net - An Overclocking Community
Thank you very much the information has been very useful
it educational for me also