Instead of price reduction I want increase

[fjanos]

In Admin Back office > Catalog > Create New Product where it says 'Reduction amount:' I'd like to add a new option 'Increase amount' with all the functionalities that 'reduction amount' has but the opposite. Instead of price reduction I want increase. Of course with date settings: 'available from -to'. Any idea how I can make that modification? Where should I start. Which functions should I modify? Since there is a shortage of prestashop documentation I don't know where to get started. I began reversing in AdminProduct.php where actually the form is and I created a "clone" of that form:

    
'.$this->l('Reduction amount:').'

                   '.($currency->format == 1 ? ' '.$currency->sign.' ' : '').'<input size="11" maxlength="14" type="text" name="reduction_price" id="reduction_price" value="'.$this->getFieldValue($obj, 'reduction_price').'" onkeyup="[removed]this.value = this.value.replace(/,/g, \'.\'); var key = window.event ? window.event.keyCode : event.which; if (key != 9) reductionPrice();" /> '.($currency->format == 2 ? ' '.$currency->sign : '').'
'.$this->l('OR').'
                   <input size="10" maxlength="14" type="text" name="reduction_percent" id="reduction_percent" value="'.$this->getFieldValue($obj, 'reduction_percent').'" onkeyup="[removed]this.value = this.value.replace(/,/g, \'.\'); var key = window.event ? window.event.keyCode : event.which; if (key != 9) reductionPercent();" /> %

I started tracking the reduction_price object and reductionPrice() function but it seems to me that it has connection with many things. Has anyone done this before or something similar? I'd appreciate your help.

[fjanos]

But I have prestashop 1.3.2 then it's a new feature in 1.4