Q. What will happen if we change front wheel to back side and back wheel to front side - Access 125?

First of all, you cannot do so, because the wheel design itself would not fit, in case of disk brake. In case of drum brake too the speedometer connection of 12' size would not fit the back. However, with all alterations if you do also... the speedometer may not show you the actual speed and mileage covered.

If you do like this then you want to face a big problem because the maximum load capacity of back tyre is greater than the front tyre so when you do like that you will have lot of problems with that

The size of tyres are calculated very pprecisely so changing it would be worst thing especially for access125 because tyre sizes are different for front and back