Dear Hadi, You may try the solution based method or just disperse the quantum dots  in the new ligand (about 5 mg/ml) and stir continuously at lower or RT overnight (or for 1 day). So if you insist on using the QDs already in OLA then you should know the concentration of the dots and relate that to the advised concentration for solid dots. This is one of the techniques, many others can be found in literature (publications) including the ones below.

S. Ananthakumar et al. Solar Energy 106 (2014) 151–158

P.Kalimuthu et al. Materials Chemistry and Physics 122 (2010) 380–385

Dear Hadi,

ligand is possible if you want them water soluble using thiolated ligand. as you are telling about sort chain functionalized alkanes u need high temperature. instead of using OA have u tried using these short chains. try these i thing it may work out. as there are reports of synthesis of QDs using these acids. if u want water soluble then try mercaptodecanoic acid.

If you plan to exchange ligands with similar head groups (here carboxylic acid), thus with similar affinity, you really need to flood you sample with the new ligands, with relatively high concentration, for long period of times, many times.

Look at: ACS Nano, 2010, 4 (2), pp 869–878 (here the replace OLA with oleylamine).

Each exchange step will occur according to a diffusive equilibrium, where you might exchange 80% during the first round, than 80% of what remains in the second round, and so on. You can monitor the removal of OLA through its C=C bond (FTIR), as the other acids you're using don't contain double bonds. However, the steric forces induced by the C=C and the long chain of OLA are what makes the particles stable in non-polar solvents. I cannot guarantee that your new ligands will prevent precipitation.

Good luck