Problem with emails when completing orders

I was asked to investigate why a website was very slow. The site sells places at a campsite which is only used occasionally by people visiting events at a nearby racetrack. After copying the site to my staging server I found it was running very low on memory, which appeared to be because it was looping through all the incomplete orders. There were over 1400 of these. The site uses a combination of WooCommerce and Events Calendar PRO plugins among others, WooCommerce is version 2.4.6.

I thought I would first upgrade to the latest versions (WooCommerce 2.6.14 at the time) which worked on my staging server, but on the live site it crashed with an out of memory error and we had to restore from a backup. So we started updating orders for past events to ‘completed’, forgetting that it would send order complete emails to people, causing confusion and worry. We then changed the text of the customer-completed-order email to a follow up sales letter, and all appeared to work well if we changed the Order Status to Completed on the Order Details page, but if we use the bulk action ‘Mark complete’ on the Orders page it sent a different email, containing very little except the original order details, causing worry and confusion yet again.

My questions are (1) what is the difference between these two ways of changing the order status to completed, and (2) how can we complete orders for past events in bulk, preferably without sending an email?

Also any advice for a better way of processing these orders in future would be welcome. I understand WordPress, but I’ve no experience with WooCommerce.

Thanks for your help.