I switched a Merc diesel to full synth at 300,000 miles. Not a problem. In fact, the turbo spooled up faster and the car became quicker by a bit.
Never, ever change viscosity in an engine that uses any kind of variable valve timing or other, oil pressure activated engine controls. The different viscosity will screw with the timing and/or other controls. The only way to "compensate" for engine wear is to rebuild the engine.
The above is, of course, null and void if you've the education of a Toyota engineer.