We have listed several types of upgrades that are available and have included the reasoning behind each upgrade.
Community 1.XA to Community 1.XB In this upgrade path the reason may be to keep the current 1.X theme and extensions in place while upgrading the backend code to a newer version of Community 1.X. This tends to be the lowest cost with the fastest schedule. This upgrade can sometimes takes advantage of newer versions of code, fixed bugs, memory utilization improvements and security upgrades.
Community 1.X to Community 2.X In this upgrade path the reason may be to get on the latest version of Magento Community and take advantage of the newer technology. The cost tends to be equivilent to building a new Magento site. The reason for this is that the 1.X theme, extensions and customizations cannot be just "migrated" over to 2.X. It's technically a new build which includes a new 2.X Magento theme. The cost savings over building a new site is that the database can be migrated over with Magento's migration tool to 2.X and, if the requirements all remain the same, saving time without having to do another deep discovery of requirements. At this stage of an upgrade it's worth considering what extensions you really need to move to 2.X. Doing a refresh on the frontend store design including adding responsive design if it was not on the 1.X site.
Community 1.X to Enterprise 2.X In this upgrade path the reason may be that your Community 1.X site is maxed out with some capabilities. By upgrading to Enterprise 2.X, you take advantage of not only the new 2.X platform but also the benefits of Enterprise. This cost tends to be almost equivilent or slightly higher as building a new Magento site. The reason for this is that the 1.X theme, extensions and customizations cannot be just "migrated" over to 2.X. It's technically a new build which includes a new 2.X Magento theme. The cost savings over building a new site is that the database can be migrated over with Magento's migration tool to 2.X and, if the requirements all remain the same, saving time without having to do another deep discovery of requirements. At this stage of an upgrade it's worth considering what extensions you really need to move to 2.X. Doing a refresh on the frontend store design including adding responsive design if it was not on the 1.X site.
Enterprise 1.X to Enterprise 2.X. In this upgrade path the reason may be that your store is ready for a major improvement or you simply want to take advantate of the new Magento 2.X platform. By upgrading to Enterprise 2.X you take advantage of the new 2.X platform. The cost tends to be equivilent to building a new Magento site. The reason for this is that the 1.X theme, extensions and customizations cannot be just "migrated" over to 2.X. It's technically a new build including a new 2.X Magento theme. The cost savings over building a new site is that the database can be migrated over with Magento's migration tool to 2.X and, if the requirements all remain the same, saving time without having to do another deep discovery of requirements. At this stage of an upgrade it's worth considering what extensions you really need to move to 2.X. Doing a refresh on the frontend store design including adding responsive design if it was not on the 1.X site.
This is a tough question to answer without having any details. Every Magento Community site is different so here are a few things to think about when asking this question.
There a many reasons to upgrade to Magento 2.X. The first is that new platform is a redesign of the platform focused on the future. Benefits can be found here https://magento.com/products/enterprise-edition. Whether going from CE 1.X to CE 2.X or EE 1.X to CE 2.X getting on the next platform version positions your business for the future and benefits of Magento 2.0.