Convert 0 -2.5 V to 0 +2.5V for measure with adc
Can anyone recommend a simple schematic / device etc which is capable
to just \$\color{red}{\text{invert the polarity}}\$ and not affect the voltage

It all depends on how close to 0 volts you want the circuit to perform linearly. If &quot;100%&quot; then you need a negative power rail. If you can live with a dead-band of a few milli-volts when the input signal is close to 0 volts then you can use an LM324 in an \$\color{red}{\text{inverting}}\$ configuration or, an LT1078 (an improvement on the LM324): -

When the input is positive, because the op-amp circuit is an inverting configuration, the output clamps at near to 0 volts. If the input is negative (below ground) then the amplifier works as a normal inverting amplifier and produces a positive output.
The LT1078 is good in this respect because its output can fall to nearly the 0 volt rail. Not many op-amps will be suitable except maybe the LM324 but it has an input offset voltage of a couple of milli-volts whereas the LT1078 has an input offset voltage of 70 μV.
So, it comes down to what compromises you are prepared to accept. The LT1078 (lightly loaded) can swing down to within 1 milli-volt of the ground rail. Only you can say if this is acceptable.
If you want to digitize high speed signals then you'll need to use a small negative rail for the op-amp to prevent output saturation delays distorting the signal.