How Do You Move From Hosting to another with No Downtime or DNS Propagation Delay?

How Do You Move From Hosting to another with No Downtime or DNS Propagation Delay?

Usually Name server changes usually take 24 to 48 hours to fully start working, which may take lot of time for DNS Propagation and website usually get down at some point,

So, how it's possible to do it without much of downtime ? any ideas ?

is the “unknown” in the path of this email, the source of the delay and does it occur prior to leaving Docusign?

Created at: 6/6/2016, 1:05:58 PM PDT ( Delivered after -13 sec )

SPF: pass

DKIM: pass

DMARC: pass

# Delay From * To * Protocol Time received
0 docusign.net CHFE28.corp.docusign.net 6/6/2016, 1:05:58 PM PDT
1 3 sec unknown → mailch.docusign.net ESMTP 6/6/2016, 1:06:01 PM PDT
-16 sec mailch.docusign.net[Google] mx.google.comESMTPS 6/6/2016, 1:05:45 PM PDT
3 → [Google] 10.36.65.162 SMTP 6/6/2016, 1:05:45 PM PDT
4 → [Google] 10.103.10.198 SMTP 6/6/2016, 1:05:45 PM PDT

Is the "unknown" in the path of this email, the source of the delay? Does it occur prior to leaving Docusign? Does it provide an oppertunity for tampering to occure? 

blockchain.info API response is coming with a big delay

I’m using blockchain.info API on my shop. Everything was fine until about 2 weeks ago. The whole procedure and the issue is: – When a customer place an order, a btc address is created and customer must send a certain amount of funds to this address. – When funds are received on this wallet, blockchain.info is sending a response to my site that funds are paid. – Automatic confirmation emails are sent to the customer when I receive this confirmation from blockchain.info.

About 10 days ago, I noticed that the “paid” reply comes with a big delay from blockchain.info. Log shows that there is a difference of about 4 hours between creating the invoice and paid responce. At the same time, I see that customer paid his order (3 confirmations) in 25 minutes (for example) after invoice was created.

As to mention, there were no such issue before. Something changed and I cant understand where.