Magento – convert amount form one currency to another currency in magento 2

Here is something that should convert the prices.
I know it's not the best way of doing it by touching the db directly, but it's fast and it works.
Please back-up your db before trying any of this.

So all price related values are kept in the table catalog_product_entity_decimal and catalog_product_entity_tier_price in case you have tier prices.
For tier prices is simple.
Just update the value of the column value with the converted value.
So today the conversion rate for 1E is 4.4193 RON.
Run this query:

UPDATE `catalog_product_entity_tier_price` set `value ` = 4.4193 * `value`;

This should solve it.
For the rest of the prices is not that easy.
First you need to identify the attribute ids for price and special_price.
For that run this query.

SELECT 
    * 
FROM 
    `eav_attribute` e 
    LEFT JOIN `eav_entity_type` et ON `e`.`entity_type_id` = `et`.`entity_type_id`
WHERE 
    `e`.`attribute_code` IN ('price', 'special_price') AND
    `et`.`entity_type_code` = 'catalog_product'

This should produce 2 rows. Take the column attribute_id values and use them in the query below. For me these values were 75 and 76.
Now run this query for updating the prices.

UPDATE 
    `catalog_product_entity_decimal` 
SET 
    `value` = 4.4193 * `value`
WHERE
    `attribute_id` IN (75, 76)

Again 4.4193 is the exchange rate.

Now you can set your base currency to RON. Clear the cache and reindex everything.

Solution:

public function convertPrice($amount = 0, $store = null, $currency = null)
{
    $objectManager = \Magento\Framework\App\ObjectManager::getInstance();
    $priceCurrencyObject = $objectManager->get('Magento\Framework\Pricing\PriceCurrencyInterface'); 
    //instance of PriceCurrencyInterface
    $storeManager = $objectManager->get('Magento\Store\Model\StoreManagerInterface'); 
    //instance of StoreManagerInterface
    if ($store == null) {
        $store = $storeManager->getStore()->getStoreId(); 
        //get current store id if store id not get passed
    }
    $rate = $priceCurrencyObject->convert($amount, $store, $currency); 
    //it return price according to current store from base currency
 
    //If you want it in base currency then use:
    $rate = $this->_priceCurrency->convert($amount, $store) / $amount;
    $amount = $amount / $rate;
     
    return $amount;

}

Here, $amount is the amount to which you want to convert.

$store is the store id, from which store’s base currency you want to convert.

$currency is the currency whom you want to convert, if you passed null then it takes current currency.