Can I change my 2018 federal return from married filing separately to married filing jointly to improve my tax situation? Will this trigger an audit?
If the other spouse agrees to file a joint return with you there is no reason you cannot amend for that purpose and it will not bring on an audit.
If the other spouse also filed MFS then you have to decide which return you will amend ... you only need to amend one and this is not a real DIY project ... I highly recommend you seek local professional assistance.
If you use the TT 2018 amendment program some entries will need to be made in the FORMS mode.
NOTE: Once you've filed your tax return as Married Filing Separate (MFS), you have three years from the original deadline to make the change to your tax return. This deadline DOES NOT include any filing extensions.